Wednesday, July 31, 2013

The Yin and Yang of July Rainfall

There are two features of the July 2013 precipitation map that pop out. The first is the unusually high rainfall in the southeastern U.S., and the other is the very dry conditions in the the northwestern U.S.

Percent of normal precipitation for July 1-31, 2013.
Credit: NOAA Advanced Hydrologic Prediction Service

The axis of the high July rainfall in the southeastern U.S. extended from northeastern Georgia through western North Carolina. As of today there are at eight CoCoRaHS stations in western North Carolina that accumulated 25 or more inches of rain during July alone, with the observer at NC-AV-4
(Linville 2.4 ENE) reporting a total of 29.24 inches of rain! Avery County, NC has a normal July rainfall of 5.89 inches and 57.39 inches of precipitation annually. This observer recorded measurable rain on 29 of 31 days during July, and 12 days with an inch or more. Through the end of July NC-AV-4 has accumulated 75.69 inches of precipitation this year, with just a little less than half the year to go. The story is similar for many of the locations in western North Carolina.  The southeastern U.S. is waterlogged, and there may not be much of a break. The current weather pattern with an upper level trough over the eastern U.S. will continue to funnel disturbances southeastward for the next week or so.

7-day Quantitative Precipitation Forecast for the period ending 8:00 a.m. EDT August 7.
Credit: NOAA Weather Prediction Center

Also, though the tropics have been fairly quiet so far we are just starting to climb toward the peak of the hurricane season, and any storms that make landfall along the southeast or Gulf coasts could potentially bring heavy rain to these same areas. With just normal rainfall the rest of the year, the observer at NC-AV-4 would measure close to 100 inches of rain for 2013.

Observed precipitation for July 1-31, 2013.
Credit: NOAA Advanced Hydrologic Prediction Service
On the other side of the country, it was a bone-dry July in the Pacific Northwest south through the northern half of California. In Oregon, the highest precipitation total for July was 0.13 inches at OR-WL-6 (Lostine 2.4 ESE) (of all stations with 31 days of observations and no multi-day reports overlapping the end of June). A number of other locations had a few hundredths of an inch for the month, and many recorded zero.  July, August, and September are typically the dry months of the year, with a normal July rainfall of 0.95 inches at OR-WL-6. In the state of Washington it was much the same story. Many CoCoRaHs stations repored no rain at all during the month, and a lucky few had as much as one quarter of an inch. The observer at WA-FY-1 (Republic 4.2 N) recorded 0.88 for July, with measureable rain on five days and 0.64 as the largest single-day amount. Normal July rainfall for this location is 1.47 inches.

Thursday, July 25, 2013

A Refreshing Change in the Weather

Cooler and drier air spread south over most of the eastern half of the nation this week in the wake of two cold fronts. The first cold front pushed through the northeast quarter of the country last weekend before stalling out. The weather did turn a bit cooler and less humid behind this, but the the real push of cooler and drier air came behind a second cold front that swept across the eastern half of the country Tuesday and Wednesday.  Low temperatures this morning were in the 50s and 60s across much of the area, and own around 70°F in the northern Gulf States.

Minimum temperatures as of 7:00 a.m. CDT July 25.
Credit: Unisys

 Dew points today were in the 50s from the Mississippi River eastward, a refreshing change from the muggy 70s that plagued the Midwest and East last week.

Surface dew point temperature at 10:00 a.m. CDT July 25.

There were some record lows and record low maximum temperatures across the upper Midwest from this cool air mass. Record low maximum temperatures in the Gulf Coast States were largely due to clouds and rain.

Last week there was a lot of hype in the media about the heat and humidity. It was hot and humid and uncomfortable, but this period of muggy weather was not that unusual for July, and fortunately it was relatively short-lived. There were a few record and near record high temperatures last week in the eastern U.S.  There were many more record high minimum temperatures last week than record high maximum temperatures as a result of the very humid air.


Thursday, July 18, 2013

Two (More) Cases for CoCoRaHS

One of the stated goals for CoCoRaHS is to have one observer per square mile in urban/developed areas, and one every six miles in rural areas. That kind of observer density would go a long way to filling the gaps in precipitation measurements. I know I may be preaching to the choir in this post, but here are two interesting examples where the presence of CoCoRaHS observers (or lack thereof) make a difference in how we interpret a situation. One of these occurred in South Carolina this past weekend, and the other occurred today near me.

Map zoomed in on station SC-PC-13
in southern Pickens County, SC
On Saturday morning the national CoCoRaHS  "dot map" was mostly gray and blue early in the morning, usually a sign that there was a very high value on the map, either a real value or perhaps a decimal error. On closer inspection I found an amount of 8.96 inches in Pickens County, South Carolina, near Clemson.  Most of the amounts surrounding the station in question had amounts from 0.50 to 2.00 inches, with the closest station was about seven miles east.

Wider view of precipitation in Pickens County, SC
A check of the observation detail for that station made it immediately clear that this was a good value.  Here are the comments from the observer:

8.96 inches of rain!
Extreme heavy rainfall. First round late afternoon. Second round began in the middle of the night with a Flash Flood warning waking me up at 4AM. Frequent cloud-to-ground lightning and strong wind. Power fluctuations with lightning. My automatic tipping rain bucket reported at 5:40AM a 1-hour rainfall amount of 3.67". Diminished to light rain at 6:30AM. **EDIT** I'm changing the flooding statement from Unusual to Extreme based on observations in daylight. Creek backing onto our property, as well as Nettles Park on the other side of the creek, has burst its banks and flooded playing fields up to the score board level! Lifetime locals claim they've never seen this in their 40-50 years.

Two lessons here.  First, comments are extremely valuable!  The second lesson is that even with 12 to 15 other observations within a 15 mile radius, not one came even close to this rain amount.  More observers would have really helped define this storm. Here is a comment from the observer (a trained meteorologist) in an email:

It really is too bad there aren't more observers because I would hazard a guess that areas in and near Pendleton, SC which is just 3 miles south of me had really heavy rain before we did.  And that's where the rain seemed to be reforming, right over Pendleton on north.  Then as the storm tried to diminish, the whole thing just shrank right over southeast Clemson.  As it was shrinking, the intensities went way up.  The lightning/thunder was incredible as well.

Case #2 is an example of a storm occurring between observers. I live in Champaign County in east-central Illinois, Despite 45 to 50 active observers in the county, not one was located under this storm.  We need more observers!

Rain shower about 3 miles NW
On my way home this afternoon I noted there were some good building cumulus clouds. About 20 minutes into my 30 minute trip, I saw a well-defined rain shaft to the northwest and stopped to snap a photo.  Once home I checked radar and it turned out this was the only shower or thunderstorm within 90 miles and about 8 miles northwest of my location. As near as I could tell this cell started to rain about 2:45 p.m.

By 3:30 p.m. the storm had intensified and was no doubt producing heavy rain. At it's maximum the storm was about 3 miles in diameter, and during the course of its lifetime (a little less than 2 hours) it moved a distance of only 2 miles. Winds from the surface up to 20,000 feet were only 5 to 10 knots.

Left:  Radar image a 3:37 p.m. CDT when storm was its most intense.
Right: Radar image at 4:19 p.m.
The cell was totally dissipated by 4:30 p.m. Champaign-Urbana is at the left edge of the image.
Interstate 74 is marked by the east to west red line.

This storm moved very little, so unless it was sitting over rain gauges when it developed it's unlikely any nearby gauges would have measured much, if any rainfall.  What about our CoCoRaHS observers?  Here is an image of the radar estimate rainfall with this cell, plotted with the location of the active CoCoRaHS observers.

So how much rain?  We have observers to the east, observers to the west, observers to the north, and observers to the south, but not one where that cell developed and died out! According to radar, someone's corn and soybeans may have received a nice 1.00 to 1.70 inches of rain. But, we'll never know for sure.

We can never have too many observers!

Friday, July 12, 2013

Westward Ho! A Potential Drought Denter

500 millibar map (~20,000 ft) for 7:00 a.m. CDT July 12, 2013
Normally weather systems tend to move in a general west to east direction across the U.S.  However,over the next five days an upper level low that was located over West Virginia this morning is forecast to retrograde, or move west, to the Desert Southwest.  That's quite a trek, and not one I can ever remember seeing before.  The system that retrograded from the Ohio Valley back to the Plains did not make as long a trip as this low is forecast to make.

The upper trough been responsible for rain in the east the past few days. Rain was falling this morning from New Jersey and southeastern Pennsylvania south along the Atlantic coast. As it this low moves west the next few days it will be responsible for unsettled weather over much of the country east of the Rockies, and will eventually help kick the southwestern monsoon into gear. The eastward moving wave along will have ample Gulf moisture available to work with. East of the wave sultry summer weather will take hold and most of the eastern half of the U.S. will be dealing with highs from the upper 80s to the 90s as an upper level ridge builds in behind the trough.. Meanwhile temperatures will be cooling in west Texas and in the southwest as clouds and rain become more prevalent.

Here is a loop of the 500 millibar level for every 12 hours from today through the morning of July 17 showing the upper low moving west into Texas and New Mexico.

Some hefty rainfall amounts are indicated in the model forecasts. Rainfall amounts may reach three inches from Oklahoma into southeastern New Mexico.

7-day Quantitative Precipitation Forecast (QPF) for the period from 7:00 p.m. CDT July 12 to 7:00 p.m. July 20, 2013

This system won't be a drought breaker, but it will put a small dent in the drought in those locations that get the rain.  The bottom line is that the long-term drought will need long-term precipitation to be reversed.

Of course, the final path of the low and its strength (and the associated weather) may change in future forecast model runs, so this rainfall forecast will likely change. If you would like to keep tack of the updates to the QPF visit the NWS Weather Prediction Center web site.

Tuesday, July 9, 2013

Air as a Fluid - Stunning Video of Marine Layer

By Brocken Inaglory (Own work) [CC-BY-SA-3.0
or GFDL (],
via Wikimedia Commons
You may have learned or heard that the atmosphere behaves as a fluid. If you have trouble imagining air as a fluid, this breathtaking time lapse video of the marine layer (see bottom of page) should put any
misconception to rest very quickly.

The marine layer is an air mass which develops over a large body of water such as the ocean or large lake. If you have ever lived on or visited the west coast you probably have heard this term. It is a common feature over the cold waters of the Pacific.

The marine layer is relatively shallow, and forms when a temperature inversion (a layer of air where the temperature increases with height) develops above the surface. The inversion can form by either warm air moving in aloft, or by the cooling effects of the ocean water. The latter is most common on the west coast where the air is modified by cold waters of the Pacific. The cooler air near the surface is more dense than the warmer air above and is trapped below the inversion. When the moisture content of the air is high enough, condensation results in fog and or low clouds. The height and strength of the inversion determines the depth of the layer and how long it may persist.

The marine layer can form well offshore and then move inland as a sea breeze develops. The lands warms faster than the water causing air to rise. Air moves in from the ocean (in this case) to replace it, advecting the marine layer into the coastal areas.

This amazing video was produced over a period of two years. Enjoy!

Adrift from Simon Christen on Vimeo.

Monday, July 8, 2013

Spectacular Hail Storm Near Calgary

Most of us are able to observer hail from a single location and never get to see a true picture of a hail fall pattern. You may have already this aerial photo of a stripe of hail covering the ground near Airdrie, Alberta, about 20 km north of Calgary. It's been on the web the last few days. This photo is a first for me - I have never seen an aerial view of a hail fall like this.

Hail swath on the ground near Airdrie, Alberta Canada. 
Credit: Cpt. Daryl Frank, Jazz Aviation
The storm hit Airdrie about 3:30 p.m. MDT. Here is a radar image from the radar near Calgary at that time. The purple colors in the cell west of Airdrie indicate very high reflectivity of the radar indicative of heavy rain or hail. Click here to see a radar loop of this storm. Hit the play button (>) at the top of the image to start the loop.

Finally, here is a video posted on YouTube of the hail storm as it occurred near Irricana, Alberta. Irricana is located about 30 km east of Airdrie. The radar image at 1640 MDT shows the cell after it hit Airdrie and about the time it was over Irricana.  Except for the noise, this looks like it could have been recorded on January 6 instead of July 6. The hail reportedly accumulated to a depth of 12 inches in about 15 minutes.

Saturday, July 6, 2013

Muggy and Wet Weather Thanks to Bermuda High

For the last week or so a southerly flow of air has persisted across the eastern third of the nation. This flow has pulled moisture northward from the Gulf of Mexico producing humid weather throughout the central and eastern U.S.  The moisture has also fueled showers and thunderstorms that have produced heavy rain from the Gulf Coast into the Northeast.

7-day precipitation ending the morning of July 6

The extent of the moisture on the surface is seen from the dewpoint chart from this afternoon. Dewpoint great than 65°F cover the southeastern third of the country.

Dewpoint chart for :3:00 p.m. CDT July 6, 2013

The southerly flow has been produced by two atmospheric features. One is the upper level trough that has been hanging around the central U.S. for the better part of a week and the other is a strong and persistent high pressure area called the Bermuda High. The Bermuda High is a semi-permanent, subtropical area of high pressure in the Atlantic Ocean. This feature is seasonal in nature. During the winter it ends to be located near the Azores (and is called the Azores High then). In the summer the center of this high tends to be located near Bermuda, and thus the name.

On this morning's surface map you can see this feature off the east coast. Note how the high extends well into the Midwest.

Surface map for 10:00 a.m. CDT July 6, 2013.

This morning's satellite water vapor image is once again rather striking. The center of the weakening upper trough over the central U.S. can be clearly seen over southeastern Missouri. The upper level ridge associated with the Bermuda High is easy to identify over the western Atlantic. The sinking air under the ridge suppresses cloudiness and dries the air, thus the dark and orange colors.  In between the trough and the ridge the southerly winds are funneling the moisture north from the Gulf of Mexico into the Ohio Valley.

Satellite water vapor image for 10:45 a.m. July 6, 2013

Wednesday, July 3, 2013

A Window on the Upper Atmosphere

The current large scale weather pattern over the U.S. is interesting in a number of ways - its persistence, intensity, and extent. In my last post I discussed how this year's pattern is similar in many ways to what was occurring last year.  I often use the 500 millibar map (this is about 18,00-20,00 feet in altitude) to show the upper level wind and weather pattern. Looking at lines on a map is one thing, but seeing a "real life" visualization of it is something else. Yesterday's water vapor satellite imagery was striking to me as you could easily see where the major weather features were across the U.S.

You are probably familiar with the visible and/or infrared satellite images of clouds or storms. Water vapor imagery is unique in that it can detect water vapor (gaseous state) as well as clouds (liquid state). The water vapor sensors on the satellite see only about the top third of the troposphere (approximately 20,000 to 30,000 feet), so dry and moist areas at upper levels can be clearly seen. On unenhanced images, white areas show where moisture is present, and grays and blacks show where it is dry. Sensors on the satellite detect the radiative energy from the water vapor and clouds. The output voltage of the sensors is proportional to the energy striking the sensor. This in turn is converted to a "brightness temperature", which is the scale use see on the bottom of the image.

Here is the water vapor image from Tuesday, July 2 at 1200UTC (7:00 a.m. CDT). This image is from the National Center for Atmospheric Research (NCAR) Real-Time Weather Data web site.  This image is color enhanced. The driest areas are in reds, and the areas of highest moisture are in violets to greens.

Water vapor satellite image for July 2, 2013 6:45 a.m. CDT

Here is the 500 millibar map for the same time yesterday.

500 Millibar map for 7:00 a.m. July 2, 2013.
Again, the water vapor image with some map features identified.

Water vapor image 6:45 a.m. July 2, 2013 with features marked.
Notice how easy it is to pick out the upper level features depicted on the 500 millibar map on the water vapor image. You can clearly see the upper ridge in the west and the trough in the central U.S. with its center in southern Illinois. Additionally, a small disturbance in southeastern Minnesota (yellow circle) clearly visible on the water vapor image is just barely evident on the map (yellow shading). This is an area of vorticity, or rotation, in the atmosphere. Very often these are the features that help fire thunderstorms. In fact, yesterday afternoon thunderstorms developed over Iowa as this feature moved south southwest across the state. Moisture associated with the developing seasonal monsoon is evident over Mexico and southern Arizona. An area of thunderstorms (green/blue) associated with a weak tropical disturbance is evident over Cuba.

Here is today's water vapor image, 24 hours after the previous image.  The center of the trough in the central U.S. has moved back west into western Missouri. The area of thunderstorms near Cuba yesterday have moved into the Gulf of Mexico.  Dry air off the east coast is associated with the upper level ridge over the western Atlantic.

Water vapor image for 6:45 a.m. CDT July 3, 2013

Monday, July 1, 2013

A Westward Shift in Summer Heat

While I was on vacation last week our weather here in the Midwest transitioned from warm and humid to cool and rainy. High temperatures the last three days here in Illinois have been in the mid 70s, and prospects through the 4th of July are for continued cool weather. The reason for the change was the development of a strong upper level trough over the central U.S.  This contrasts remarkably with last year when the central U.S. was baking under a strong high pressure ridge and widespread highs in the upper 90s to 100s were recorded as we headed toward the 4th of July.

Mean 500 millibar map (~18,000 ft) for July 1-4, 2012

Mean 500 millibar map (~18,000 ft) for June27-30, 2013
The upper atmospheric pattern is much the same as it was this time last year, except the ridge is further west. The dry, sinking air beneath ridge is helping produce record temperatures throughout the southwest U.S., while a large trough over the central U.S. is responsible for generally cooler than normal conditions. Last year much of California and parts of the southwest were cooler than normal to near normal the last week of June and early July.

Record highs and record high minimum temperatures have been recorded through the west the past week.  Death Valley, CA reported the following high temperatures the past three days:

June 29       125°F
June 30       129°F
July 1          127°F

The temperature of 129°F on Sunday new monthly record and a tie for the hottest June temperature ever recorded in the U.S. This is preliminary and will be validated by a team of meteorologists and climatologists.

After three consecutive days with highs over 120°F, Needles, CA reached "only" 115°F today.

Record temperature locations for the last week. Credit: HAMWeather

It looks like the upper ridge over the west will start to break down by the end of this week. The will mean moderating temperatures for much of the west, although temperatures are still likely to run above normal.