Brutal cold coming late Monday, then a pattern change

cwg_JunkerIf you thought today felt cold, just wait til next week.  The coldest air mass since at least 2009 blasts into the area Monday.  But, by late next week, there are signs the pattern will start to change to a milder one.

The next two weeks in brief…

Prior to Monday’s cold outbreak, a storm will pass to our north bringing briefly milder air Sunday and rain showers Sunday night.  As Monday’s  arctic front comes through, it may  produce a dusting of snow early in the day.  Usually chasing snow with arctic fronts is fruitless but today’s GFS is predicting the possibility.

In the wake of the arctic front, single digits are a good bet Tuesday morning and a few locations could drop below zero.  Temperatures may not surpass the teens Tuesday afternoon.

As the cold wave starts to ebb next Thursday, a weak storm system may approach.  Right now it looks like a rain storm but enough stale arctic air may linger that it needs to be watched closely.  A mixed precipitation event is still a possibility as arctic air is sometimes hard to displace.

But then the pattern may well change to a warmer one. By January 10 or 11, we may rebound into the upper 40s and the pattern is one that sometimes supports a day or two well into the 50s.  Warmer than normal temperatures combined with a generally unfavorable storm track for winter storms suggest lower than normal chances of getting a significant snowstorm during the week ending January 17.

How confident are you about the arctic air mass coming next week and on the warm up to follow?

I’m very confident about the cold outbreak and am pretty confident that there will be a warm up towards mid-month.

The plume diagram below shows how much agreement there is amongst the ensemble members (different model simulations) about the cold outbreak.

temp-wes

Remember ensemble members are just identical simulations but have their initial conditions tweaked a bit since we can never be totally confident in the initial state of the atmosphere.  The ensemble members suggest that chaos will not be a big player messing up model temperature forecasts.  All have the temperatures dropping into the single digits early next week and quite a few have the area around Sterling, Va. dropping below zero.

They ensembles are also in decent agreement that the pattern starts to moderate by January 10 or 11.

Why are you expecting the pattern to shift to a warm one by mid month? 

epo1-wesThe upper level pattern across Alaska and western Canada is predicted to change significantly from the pattern that is currently delivering the cold.  Above normal heights (pressures) have been located over Alaska for most of the winter.   This ridge has redirected the jet stream well north of its normal location across Alaska,  and to the east of this ridge there has been a big dip in the jet stream that has produced northwesterly flow aloft across western Canada.  The northwesterly flow has directed cold air from northern Canada into the U.S.  But in another week or so, we are losing the arctic connection.

The pattern at least temporarily appears to be transitioning to the positive phase of the East Pacific Oscillation (EPO).  The phase is associated with below normal heights across Alaska (see figure to the right, top panel, blue colored area) and above normal heights just to the south (red shades over the central Pacific) .  That configuration typically produces a stronger upper level jet across the northern Pacific into Canada flooding the continent with Pacific air which tends displace the really cold air across southern Canada and the northern tier of the U.S.

The bottom of the two figures (to the right) illustrates the typical temperature distribution across North America in the positive EPO pattern.  Instead of the Great Lakes region being frigid like it has been, temperatures across that region with a positive EPO tend to be warmer than normal (orange and red shades).

The January 11 through 16 pattern

Last night’s European ensemble mean for the 5-day period ending January 16 has all the characteristics of a positive EPO pattern.  If you compare the 500 height anomaly pattern below with the EPO pattern above they are almost identical.

(WeatherBell.com)

(WeatherBell.com)

Instead of having a big ridge (red area) over Alaska and western Canada that we’ve had almost all winter, heights at 500 mb for the period are forecast to be below normal.  The lack of the strong ridge means systems track from the Pacific eastward across the country.  That tends to keep the arctic air at bay.  Note how similar the forecast pattern of 850 temperatures is to the composite.   The red areas that indicate above normal temperatures are almost identical to the EPO composite pattern.

Another aspect of the forecast pattern that makes it a discouraging one for snow lovers is the above normal heights at 500 mb (18,000 ft) across the East.   Such a pattern suggests storms will have a tendency to track to our north. Furthermore, the lack of any real blocking suggests any cold shots in the east would be very short-lived and probably on their way out before any storm impacted our area.   Such a storm track also dictates that our winds will have a southerly component as lows pass to our north.

The deep southwesterly flow also argues for the potential for a day or two to get well into the 50s and maybe even touch 60.  The southwest flow should also deliver quite a bit or moisture which probably will result in a wetter than normal period.

For cold lovers, there is a little hope.  The GEFS ensemble mean last night suggested the positive EPO pattern may be fleeting.   It is not as warm-looking in the long ranges as last night’s European model.

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