Monday, March 4, 2013

WX UNIT DISCUSSION 03/04/2013 0730Z

Monitoring Winter Weather Development Across Central United States




Progressive and broad upper level flow across the continental United States is becoming more zonal with time as the trough in the eastern half from earlier today is progressing eastward and the upper level ridge breaks down in the central United States.   A broad based split flow regime is now across the Rocky Mountain region moving into the Great Plains with a strong broad zonal flow of around 100KT coming in off the Pacific Ocean across southern California and the Great Basin into the southern Plains of Texas and Oklahoma.  The northern stream is wrapping around a vortex positioned along the U.S. and Canada border over norther Montana, southwestern Saskatchewan and southeastern Alberta.  Shortwave flow associated with this vortex noted from the western Dakotas westward into Idaho and then arcing northwestward across Washington State into the offshore region of British Columbia.  The shortwave has not amplified much but gives the appearance of a slight positive tile while the upper ridge from earlier today has flattened a bit at the H500/H300 levels.  

At the surface there is a broad area of cyclogenesis orientated north to south along the eastern high plains now drifting more into the central Plains from the panhandles of Texas and Oklahoma northward across western Nebraska and into the western Dakotas and eastern Montana.  Three primary areas of low pressure with the strongest noted between Enid, Oklahoma and Gage, Oklahoma, another in extreme northwestern Nebraska around Alliance and the third low located in western North Dakota.  

The southern low exhibits a trough extending southward into Mexico while the Nebraska low is exhibiting a trough extending northward through the other lowest pressure plot in western North Dakota with the trough turning northwest from there into northeastern Montana.  The Nebraska low exhibits another trough line from near Alliance (AIA) southwestward into southeastern Wyoming and then westward into the Casper, Wyoming (CPR) area.  Between the Nebraska surface trough and the North Dakota and northeastern Montana troughs there is a pronounced ridge of high pressure from western South Dakota westward along the Wyoming and Montana border which then curves northwestward across the Rocky Mountain range or western Montana northwestward into British Columbia.

Strong isobaric gradient from the surface to mid levels with the shortwave across western Montana where strong winds have been noted across central and western portions of Montana today.  Those strong winds started as a quasi-Chinook wind event earlier today with some compressive warming noted earlier for a time in Montana but have been undercut now by a colder core of air wrapping around the Montana vortex.   Still some fairly strong winds noted at several stations including at Billings (BIL), Livingston (LVM),  Lewistown (LWT),  Great Falls (GTF), Helena (HLN), Butte (BTM), Havre (HVR), Cut Bank (CTB), and Bozeman (BZN) areas with some gusts into the 35 to 40 MPH range.  Strong winds also noted across southeastern Alberta and southern Saskatchewan.   

All these ingredients are making for a very difficult forecast as the system exhibits a winter storm potential but the target snow area will be difficult to determine.  Apart from using any model consensus my earlier discussion is right on target as this system is a blend of not only an Alberta Clipper system but also a Western Type Low and even some shades of a Panhandle Hook.  Let me explain this below:

It should also be noted that this system has a narrow band of H850 to H700 frontogenetic forcing going on along and juxtaposed with the entire Missouri River Valley Region from the STL area northwestward though Iowa, southwestern Minnesota and into the Dakotas.   This boundary is the primary focus for the development of a wintery mix of precipitation that is ongoing at the moment in a narrow band extending across central Iowa, southwestern Minnesota, northeastern South Dakota and into North Dakota and northeastern Montana.  

I want to point out the strongest surface low right now in northwestern Oklahoma because it has some strong surface features for this time of year but is in a very dry air mass lacking anything but a trace of Gulf Moisture transport even at the H925 and H850 levels.   From the surface low that is to the wast of Enid, Oklahoma (END) centered right around Gage, Oklahoma (GAG) is a dryline/dry-wedge to the south-southwest along with some impressive temperature contrasts surrounding this low and very little moisture flux being transported northward.  Strong winds are also noted down in this region as this low center intensifies around 997MB.  I'm going to plot this rather that explain it below.  As you can see this is so far nothing but a dry windy system with some dew point density and temperature boundaries that are wrapping around this system:

Could it be that this southern branch will actually take over and form an impressive storm for the eastern United States for late Monday and into Tuesday?  It appears worth watching to me to see how this unfolds over the next 12 to 18 hour period. I could continue to go into all manner of details on this but I'm going to hold off an submit to COMPLETELY SHORT TERM MONITORING OF THIS PARTICULAR SYSTEM.  All manner of lengthy discussion is not worth the time as this does not appear to be a dangerous winter storm however it will cause the usual travel difficulty with considerable snow and the blowing and drifting attributes.  

Meanwhile we have this boundary to contend with in the Missouri Valley Region producing an area of light snow and some mixed precipitation right now but it does not appear to be too intense save for a few stronger isolated snow bands.  The heaviest activity is noted right in the Des Moines, Iowa (DSM) vicinity and in east central North Dakota:
From here and based on the trends I don't see a major snow producer given the fact that the dynamics of this system have lost some moisture now until it moves east of the Mississippi River and can tap into some Gulf moisture but with lack of southerly flow at the moment it seems the southern branch of this storm will remain a dry windy event and the northern branch will be an aggressive Clipper like system with about a 4 to 6 inch variety snow event in a narrow configuration with some localized higher amounts possible in a few counties across portions of North Dakota, Minnesota, and Iowa during the next 12 hour period.  The caviat comes late Monday however depending on the speed of this Montana shortwave and how it transitions into the southern stream and also if it can realize some increased moisture transport from the Gulf of Mexico.  More information on this will be forthcoming tomorrow afternoon when a better look at data and trends is realized. 

How this system progresses and redevelops in the mid-Mississippi Valley and Ohio Valley could spell some considerable higher snow amounts for places like Milwaukee (MKE), Chicago (ORD, MDW), Rockford (RFD), South Bend (SBN), Toledo (TOL), Fort Wayne (FWA) and Indianapolis (IND) eastward. 

For now KEEP UP WITH THE FORECAST especially across areas such as NORTHERN ILLINOIS, SOUTHERN WISCONSIN and eastward as this system needs to be monitored for redevelopment across the mid-Mississippi and Ohio Valley areas late Monday and early Tuesday.