WKU Meteorology

A squall line has been very slow to move southeast. Gusty winds approraching severe limits and small hail are possible. Helicity values are still very high and rotation may quickly form in any storm as well.

Thanks tor the storng overnight LLJ a squall line deelvoped to our west and has moved into areas to our north and west. A tornado has already touched in Madisonville,KY this morning. A extreme amount of shear exisits ahead of the squall line (helicity values over 700 m/s^2). This promotes the chance that any storm may turn tornadic. Strong gusty winds are the other main threat. Should a stronger storm track over Warren county some hail may occur. This squall line will redelevop and strengthen to our southeast as the last update indicated. Southeast KY and northern TN will have a very nasty day and the SPC has upgraded these regions to a moderate risk.

A warm front is still expected to move through overnight. A strong Low level jet (LLJ) will transport moisture into the boundary and provide the lift needed for a period of showers and storms in addition to keeping temperatures rather steady around 60 overnight. Southerly winds will increase as well. Along this warm front, shear will be more than sufficient for severe weather. The majority of this activity I expect to be elevated with means just heavy rain and thunder.  However due to the large amount of shear any isolated storm which does become surface based overnight may produce a tornado.

The question is can enough instability materialize for some severe storms Wednesday afternoon with the cold front?

Wednesday afternoon the cold front moves through and will likely develop another round of showers and storms. With daytime heating and a continued transport of warm unstable air into the region I do think at least marginal instability should be present across the region. By early afternoon though the front could very well be closing in on our region. The best lift for storm development is just ahead of the front making the most likely regions for round 2 Wednesday afternoon further southeast. Storms will have a damaging wind/ isolated tornado threat through the afternoon and early evening. Regions ahead of the front should see a temp spike into the 70′s as well.  I’ll update any warnings overnight tonight as needed. tomorrow morning we’ll use the RUC and HRRR to actually see where the front will be in the afternoon and pinpoint the exact regions that will see storms. At this point it is to early to call if the front will be through BG before thunderstorms form.

Modeled rainfall on the GFS is overdone due to convective feedback in my view. The NAM, EC and SREF are much less than the 12z OP GFS. Overall rainfall should be from 1/2 to 1 inch perhaps some slightly higher totals in storms.

Friday threat looks more favorable for timing across the region. I’m suspecting another squall line event in fact instability may be a touch more than this Wednesday event. I also feel either Sunday or Monday we’ll be stuck with 40′s for highs and a rain/snow mix. I’m seeing other forecasts back off of 60 and sunny for this time frame.

1. After starting out around 40 Monday temperatures should rebound into the low and mid 60′s for highs. Will trend above guidance with temps. A mix of sun and clouds should also be present with a southwest wind of 5-10 mph.

2. Tuesday the southerly winds should increase perhaps on the order of 10-20 mph. Cloud cover will also increase ahead of a warm frontal boundary. An advection of some instability and moisture should occur but mainly Wednesday. The threat for severe weather Tuesday night looks very similar to last Thursday, marginal. Showers will be likely with a few thunderstorms. Storms may have gusty winds. Any increase in the modeled instability may bring a severe threat into the mix, however this appears unlikely at this time for Tuesday night. Timing is not favorable for severe weather being after midnight Tuesday night for the highest rain chances. The system passing well north of the region on Monday should be culprit in the limited northward advection of moisture and in fact may send in some low level cold air keeping lows in the 30′s Tuesday. Highs should reach the 60′s.

3. A strong LLJ will keep temperatures in the 50′s Tuesday night. The cold frontal boundary will be slow to cross the region. I suspect a temperature spike occurs ahead of the boundary with continued strong southerly winds. Highs will top 70 perhaps reaching the mid 70’s. Along the cold front I suspect enough shear and instability will be present for another round of showers and storms this time with a severe threat. Damaging winds and isolated tornadoes would be the main threats However the front may pass east of east of our region before peak heating hours. Any changes in timing could alter the high temp and instability advection into the region Wednesday.

4. Don’t expect much of a cooldown behind the front. A mostly sunny day should occur with lows in the 40′s and highs in the 60′s. The next severe threat with another front moves into the region Fri/Sat.

The snow totals for the system that affected Kentucky last weekend have been compiled and are below (Courtesy NWS Jackson):

Comparing the predicted values to the actual observations, it seems predicted amounts were pretty similar.  What can be noted is the thin axis of heavy snowfall that fell through the area.  Predicting this axis of snowfall was the main problem for the event, and while totals were similar to predicted values, the axis of heavier snowfall fell well to the east of the predicted amounts.

Inclement weather and cooler temperatures heading into the weekend are possible as an upper level system will begin to quickly intensify throughout the day Thursday.  Positive vorticity advection as well as a favorable upper level air environment will help continue to deepen the upper level low as well as begin to develop and mature a low pressure system at the surface.  Warm air advection will quickly being to increase throughout the day on Thursday, and will bring highs to the lower 70’s.  Ahead of the surface low, ample shear will be in place in response to strong wind flow throughout the middle to upper levels of the atmosphere.  Along with a phasing of the jet stream, this will also support intensification of the surface low.  Uncertainty remains about the severe potential of this system due to the disagreements in model runs, but models have similar features that suggest the main threat with the system will be damaging winds and small hail given the lack of instability present.  Investigating helicity does show that enough spin could be in place throughout the atmosphere to produce isolated tornadoes, but the main threat should be the possible squall line with damaging winds embedded.

What makes this system the most dangerous is its forecasted time of arrival between the overnight hours of 9pm to 12 midnight Thursday.   Be sure to be aware of the threat and have weather radios close by.  The low for early Thursday will hover around the mid 40’s. Updates on the potential severe weather will be posted as needed.

Friday, temperatures will remain around the lower 40’s for the low, and increase to the lower 50’s throughout the day for the high.  Later into the day, cold air advection will begin to pull cooler air into the region, bringing a slight chance of snow flurries into the afternoon hours Friday.  No accumulations expected.   Saturday, Lows will be around the freezing mark, with highs struggling to get out of the 40’s.

Sunday looks to be much like Saturday with a low around the upper 20’s, and high of 41.

With the above average temperatures the 2011-2012 winter season has been associated with, its been difficult to remember that it actually is winter.  Even with temperatures currently above average in the mid 50’s, the first major winter storm looks to affect Kentucky starting early Sunday with snow totals over 5″ possible in the higher elevations of eastern Kentucky.   While models still differ a bit as far as totals go, there is little disagreement that snow will with be the main form of precipitation in the early hours of Sunday morning throughout the rest of the day in Warren County.

The following graphic courtesy NWS Louisville depicts estimated snow totals with the storm system:

Although I do mostly agree with these snow totals, there is certainly an elephant in the room.  NAM depicts an area of strong vertical velocity over the Bowling Green region, and if this feature does verify, we may be in the higher end of expected snow totals (closer to 3″) opposed to the likely 1″ prediction other models are showing.

The following graphic shows the aforementioned vertical velocity feature:

Not only that, but temperatures have been above average for a few days (highs around 53°F since February 15th).  This will make for a warm surface, so what will probably be the main factor in snow accumulation in Warren Co. will be can the snow fall rate overcome the rate at which snow will be melting. If snow can fall fast enough to cool down the surface temperature sufficiently, we will certainly see more accumulations.

This is certainly a tricky forecast, so ultimately only time will tell.  Nearly 12 hours before the event will unfold.

Update: 18Z NAM model run depicting the area of enhanced vertical velocity to the south and east of Warren County, so forecast confidence leaning more toward < 2″.

Its been quite a change in scenery over parts of the southeast as “Old Man Winter” finally takes his toll. Temperatures have plummeted well below freezing in some parts of the country and as odd as it seems parts of northern Florida have seen below freezing temperatures. Ocala, FL dipped down to a chilly upper 20s, where normals are in the mid 40s.Below is a current temperature map taken at 12PM.

Nearly half the country is at or around freezing temperatures; however, these cooler temperatures are expected to regress back towards the north by the middle of the week.

The chance for snow still sticks around ever slow slightly later this evening but by tomorrow temperatures will rise leaving behind rain as the primary form of precipitation. For Tuesday and Wednesday–temperatures will be on a climbing kick while the chance for precipitation decreases. This will bring partly cloudy skies and mid 40 to 50 degree temperatures to the area.

Highlights
Seasonal temps through Fri. with a small chance light precip Friday.
Ridging in the Western U.S. to allow arctic air to invade the Eastern U.S. resulting in below average temps. this weekend

Forecast
High pressure at the surface and zonal flow aloft, will keep us dry and seasonable with highs in the mid 40’s and lows in the upper 20’s on Thursday and Friday. On Friday a wave ejecting out of the southwest will bring us our next system with a chance of light rain on Friday, ending with a chance of light snow Friday night. Little impact is expected with this system, as it is moisture starved and if there is any precip the change over to snow will be brief as the system’s attendant cold front rapidly swings through. The bigger story is the much cooler temperatures this weekend as Canadian air comes to the region with highs in the mid 30’s and lows in the upper teens to lower 20’s. Brrr!

Highlights
• Drying to continue, warmest temps on Tue.
• Pattern change with a slight chance of drizzle/flurries late Tues. night
Forecast
The upper level low that has kept our weather cool and damp, will be moving off to the east to start the week. Dry northwesterly air will move in on Monday keeping us mostly clear with a high of around 53. As high pressure returns to the region, good radiational cooling will allow temperatures to dip below freezing Monday night with a low of around 30. As our next system approaches, Tues will be marked by increasing clouds with mostly sunny conditions the majority of the day and a high around 54. Tues. Night the pattern changes as a trough starts to dig in to the Western U.S. Initially, on early Wed., a dry cold front will bring a small chance of light precip Tues. night with lows near freezing. Wed will be cooler and 44, before temperatures moderate with another cool shot of air this weekend.