Wind Chill Advisories up for parts of the area overnight; a chilly week ahead

2015-02-22-1152-THREAT MATRIX_WIND CHILL

The combination of very cold air moving into the area and light to moderate winds will result in dangerously low wind chill values tonight.  The coldest period will be between midnight and 9 AM on Monday.  While all areas will have dangerously low wind chills, the advisory covers just part of the eastern forecast district. Each National Weather Service office has its own regional criteria for issuing winter weather (wind chill, winter storm) advisories, watches and warnings. As might be expected, the thresholds are lower for southern and eastern areas and higher for western and northern areas. That’s part of the reason some areas are included while others are not even though wind chills will be more or less similar (dangerously cold) for the entire area.   It will be important to bundle up especially for kids waiting for the bus, scarves, hats, mittens or gloves.  Although Monday morning looks the coldest, Tuesday and Friday have the potential to be other mornings of particular concern.

Here’s a look at the advisory area:

2015-02-22-0717_WIND_CHILL

 

This map shows the expected wind chill around 7AM Monday morning when most people head out to work or school:

2015-02-22-1121_WIND_MIN-MON

This are the forecast overnight lows for the region tonight–not the coldest night we’ve seen but still very cold by late February standards.

2015-02-22-1121_TEMP_MIN-MON

This map shows the wind speed (in knots) and direction (northerly) around 7 mph. One “knot” is 1.1 mph (so 10 kts= 11 mph).

2015-02-22-1121_WIND_SPEED-MON

Tuesday morning will see readings in the mid teens west and mid single numbers (above zero) east.  Winds will be southwesterly by that time and around the same speed as Monday morning so we might be looking at another dangerously cold wind chill morning in the east (whether or not an official advisory is posted).  Wednesday and Thursday will be more seasonable (lows in the 20s) but Friday morning has the potential to be another chilly morning with wind chills once again a possible issue.

Update on the week ahead – Temperatures:

There’s really been no significant change in the outlook for continued cold weather as we head through this the final week of February and meteorological winter.  We’ll see two main shots of cold this week.  The first is already in the area, the second arrives Thursday.  We’ll take a look at the model ensemble charts for this evening through next Sunday evening. These ensemble charts are made up of many members (variations of the model running with slightly different physics weighting and the same starting conditions). This gives us an average to go with rather than just one solution.  This is usually a more reliable indicator of forecast conditions. The important things to remember with these charts is that:

(1) They are in degrees C so 1 degree below average equals 1.8 degree F below average.  This means if the chart reads -10C that equates to -18F below average.

(2) These charts are the departure (or difference from) average NOT the actual air temperature.

(3) Averages are different for each area +10F from average means a whole different story in North Dakota than +10F from average in Texas.

(4) The charts are shown at 6PM local time, so for simplicity’s sake let’s consider this the difference from the average high temperature for the day.

(5) Average highs this week (Topeka, KC and STL) are 45 to 50 so let’s make 47 the ‘average’. 

(6) This model factors in snow cover (you’ll see a cold bulls eye over SE CO/SW KS/Panhandles) so the model “sees” the air is chilled by the snow cover. 

You can see the cold today relents somewhat by Tuesday as mild downslope flow moves into the far Northern Plains, then a secondary cold shot arrives quashing the warmth and rolling south Thursday/Friday before temperatures again ease next Saturday.  The model is “seeing” some sort of storm system on Sunday as it hints at southerly flow and milder air heading up into the Mid South and the colder air backs west into the Plains. At the worst, this model ensemble gets part of the area around 13C below average (Monday).  Using the conversion, 13C  X 1.8= 23.4F, so the average high (47) subtract 23 equals a high around 24F Monday.

2015-02-22-1200-TEMP-A-D00 2015-02-22-1200-TEMP-A-D01 2015-02-22-1200-TEMP-A-D02 2015-02-22-1200-TEMP-A-D03 2015-02-22-1200-TEMP-A-D04 2015-02-22-1200-TEMP-A-D05 2015-02-22-1200-TEMP-A-D06 2015-02-22-1200-TEMP-A-D07

Update on the week ahead- Precipitation

There appears to be at least the possibility of another light/nuisance type snow midweek.  This would be with a weak clipper low which forms on the leading edge of the next Arctic air mass. This morning’s guidance takes the low out of the Northern High Plains area and moves it southeast dissipating it by the end of the week.  This results in a stripe of light snows northeast of the weak low’s track.  These light snows steadily diminish in all the solutions as they move southeast. There’s a general consensus of a track from the Northern High Plains to OK/KS by three of the four models shown here.  The Canadian model is alone in forecasting a much more eastward track, from SD to W KY. With the exception of the Canadian model which has shifted notably north and east, most others have remained steady in their respective solutions today.

2015-02-22-1247-2212ZECM-CLIPPER  2015-02-22-1247-2212ZGFS-CLIPPER 2015-02-22-1247-2212ZNAM-CLIPPER 2015-02-22-1247-2212ZGEM-CLIPPER

As mentioned, this is just another light/nuisance snow event (1-2″ at best–and maybe more on the order of a dusting to 1″) and currently the western district would be most favored for this, with flurries possible almost everywhere else. It was mentioned in yesterday’s blog that with the exception of SEMO/SW IL, all the major reporting stations in the east and western forecast districts have had nothing more than these types of events all year long. Kansas City, Topeka and St. Louis HAVE NOT seen a calendar day with more than 2.9″ of snowfall this winter snowfall season.

If the Canadian model is on to something, then that opportunity for the light snows would be more focused on the eastern district with the western district likely dry, breezy and much milder than currently forecast.  That’s because as with most clippers, there will be a slug of warm dry air to the south of the incoming Arctic front and to the west of the low’s track.  That would mean currently forecast highs in the mid 30s to near 40 for KC to Topeka could be as much as mid 40s in KC and near 50 in Topeka.  The just-in as this is typed latest high resolution NAM model remains steadfast in a western track and is in fact just a few miles further west.

Next Weekend/first few days of March:

There’s almost NO confidence in one model or model solution as they continue to bounce around. Yesterday, we talked about a universal southern shift in all the available models.  Today, that’s not so at all.  They are all offering different takes.  Take a look at this panel for this model, which zeroed in on SEMO/SW IL this time yesterday and then has decided to refocus on NW MO today.

2015-02-22-1247-ECMWF

 

The other models are similar in their changes.  The GFS for example, has flip-flopped from south to north and now back to south again, focusing snow on SEMO/SW IL after focusing on KC earlier today, focusing on SEMO/SW IL last evening and KC again yesterday afternoon. The Canadian model bypassed the entire region yesterday and today it blankets the entire area with moderate snows.

Southwesterly flow is expected aloft (a warmer flow aloft) so there is the real possibility this would not be “just” a snow producer — it might well be another mixed bag event with rain/freezing rain/sleet and snow depending upon how deep the warm air aloft is with this system and how much surface warm air is wrapped into the low.  The models discussed here in this section don’t have a good ability (or any ability) to resolve those details 7+ days out.

The system would supply the upper level forcing to generate this system is currently over northeast China, and will have to cross the Pacific, arc north into Alaska and then dive southeast to the Southwestern U.S. in the next week before it begins to be sampled by the upper air network. You may hear about snow and rumors of snow through the opening days of the week, but we won’t have a better idea if or when any such system will materialize until mid week at the earliest. IF it becomes a distinct possibility (and doesn’t turn out to be a phantom storm) it will likely be one of those situations where forecast precipitation types and amounts are prone to frequent change as the days leading up to the system roll by.  We’ll monitor the situation and update the blog midweek if needed.

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