Ramblings of a Graduate Student

The Hydrometeorological Prediction Center’s (HPC) 5-day forecast once again calls for a lot of rain over a very large area. This is very similar to the past week. A tropical system will move from the Caribbean Sean northwestward into the central United States and interact with a very slow moving cold front. Expect a lot of rain with very heavy rain rates. However, the threat for widespread severe weather should be minimal.

With all the heavy rain that fell over Kansas, Missouri, Iowa, and Oklahoma this last week, the watershed is handling a large amount of water already. This water is currently flowing downstream toward the Gulf of Mexico. With the additional rainfall forecast to fall over a watershed that is currently transporting last week’s rain, caution should be used on all rivers from the central plains down toward the coast.

Now that former Hurricane Alex has made landfall, focus now turns to what happens to the remains of Alex. This is important because the locations in the path of Alex’s remnants stand to see several inches of rain and potentially even more should the remains stall over any location. This is because during the life cycle of a tropical wave (particular tropical storms and hurricanes), vast amounts of water vapor was released into the troposphere. This results in a lot of moisture for sources of lift, such as the tropical wave itself, to interact with.

The Hydrometeorological Prediction Center’s (HPC) 120-hour (5-day) forecast for precipitation has a large swath of relatively high values of precipitation forecast. This forecast is based on the moisture associated with Alex’s remnants interacting with a slow moving cold front (another source of lift). If this forecast pans out, the Fourth of July holiday weekend will be a soggy one for people in the central United States.

VORTEX 2 attempted to operate in the Texas panhandle this afternoon on a thunderstorm that produced a short-lived tornado and 6″ diameter hail. The hail was so large that it smashed through the vehicle’s windshield. Yikes! Unfortunately (or maybe in this case fortunately) for VORTEX 2 the thunderstorm weakened substantially before they were able to deploy. Additionally the storm began to turn into a huge thunderstorm complex resulting in a lot of heavy rain.

Several hours later, the heavy rain threat is continuing to increase as the number of thunderstorms increase across the central United States. These thunderstorms are tapping into an extremely moist airmass across the southern United States, so they will be efficient rain makers! This is part of the heavy rain threat forecast by the Hydrometeorological Prediction Center (HPC)discussed briefly in last night’s post.

I do not have a VORTEX 2 updated tonight because my colleagues in the VORTEX 2 Operations Center forced me to take a much needed night off. I really needed it!

With that said, tonight’s blog depicts the Hydrometeorological Prediction Center’s (HPC) 5-day forecast for precipitation. Please note the widespread heavy rain forecast across the central part of the United States. After being outside this evening and experiencing the warm, moist airmass that is in place across the United States, I am pretty sure a forecast like this will come to pass. The tragic events in Arkansas today highlights the need for people to be cognizant of flash flooding. The HPC 5-day forecast certainly seems to indicate that the potential for flash flooding will continue in the near-term.  Remember,“Turn Around, Don’t Drown”

Day 7 of VORTEX II was used as a travel day in preparation for potential targets on Sunday and/or Monday.

Above is the Hydrometeorological Prediction Center’s 5-day precipitation forecast.  As you can see, they are highlighting the central plains (I-35 corridor) for very heavy rain over the next five days.  Currently, it appears this event will pale in comparison to what happened last weekend in the Nashville, TN area, but will still be a lot of rain for areas.  At this point it appears that the precipitation will mainly fall from repeated thunderstorms early next week.  The potential will exist for severe weather and this will most likely be targeted by VORTEX II.  Tomorrow and/or Sunday I’ll post a forecast discussion focusing on the severe threat for the next few days.

For the past few weeks I’ve alluded to the fact I will have an extremely active spring.  I’m involved in both VORTEX II (although my role has continued to evolve over the last few days) and the Experimental Forecast Program (a joint experiment hosted by the National Severe Storms Laboratory [NSSL] and the Storm Prediction Center [SPC] and held in the Hazardous Weather Testbed [HWT; I’m the one in the blue shirt in the image on the upper right!)  This year’s experiment will have collaborators from the Aviation Weather Center (AWC) in Kansas City, MO and the Hydrometeorological Prediction Center (HPC) in the Washington D.C. / Silver Springs, MD area.

In preparation of this year’s experiment I was on a teleconference this morning with HPC and SPC so tonight I thought it appropriate to display a graphic produced by HPC.  Above is the forecast precipitation totals over the next 5 days.  As you can see, heavy rain is forecast for places that were hit hard by last weekend’s tornado outbreak.  This does not bode well for cleanup efforts.  This heavy precipitation is the result of another slow moving trough (#4 in last night’s post) that will move through the southeast late in the week into the weekend.

Anyone who might be a storm chaser might have been acting somewhat depressed of late.  Chances are this is because they have looked at the extended forecast and noticed that there are no “big chase days” forecast on the horizon.  In fact, it’s been a relatively quiet year for tornadoes (and severe thunderstorms in general).  And this pattern doesn’t show signs of changing any time soon…

The image at the top (and annotated below) is taken from today’s 12 UTC (7 AM CDT) run of the Global Forecast System (GFS).  It is a 132 hour forecast, valid 00 UTC Sunday, 18 April (7 PM CDT, Saturday, 17 April), of 500 millibar heights and winds.  As previously discussed, this is a good level of the atmosphere for meteorologists to evaluate when looking for troughs (cool and stormy) and ridges (warm and sunny).

The first thing that should jump out is the fact that most of the country is being influenced by a large ridge (the mountain looking thing in the black lines).  Areas that are underneath this ridge would expect (generally) sunny skies and warm conditions.  However, not everyone will get to experience the ridge in this manner.  The northeast United States is forecast to experience a shortwave trough (closed low) that should keep the weather relatively cool and unsettled with periodic rain and showers.

Also, to the south of the apex of the ridge discussed above, a shortwave trough is forecast to be present.  Essentially what is happening is that the shortwave trough is trying to move through the large CONUS wide ridge.  This typically leads to slow progression of the trough, with wind speeds within the trough being weaker (slower) than normal.  These weak mid-level winds often result in slow moving non-severe thunderstorms that produce heavy rain (assuming moisture is present). I’ve circled in green (below) the areas that would be most likely to experience this type of thunderstorms, based on this specific forecast.  Although not typically widespread, severe thunderstorms are possible in this kind of pattern with severe thunderstorms that do occur likely being of the short-lived variety with hail and wind the primary severe threats.

(Note, in the image below, trough axes are indicated by dashed black lines and ridge axes are indicated by solid black zigzag lines.)

Keeping in mind everything mentioned above, below is the Hydrometeorological Prediction Center (HPC) forecast of total precipitation for Thursday nigh through late Saturday afternoon.  Notice that the area of maximum precipitation in this forecast is roughly coincident with the area I circled above.  This would suggest a prolonged period of heavy rain for the western southern plains over the next week.  We’ll see.