Long-range forecast: A cooler than normal end to winter

March 7
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A colder than normal pattern is in the works between now and the official start of spring.  That may discourage people rooting for warm weather but may not be quite as bad as it sounds.  A couple of days well in the 50s or even low 60s are possible next week.  Then a storm around the March 12 will usher in the next period of colder than normal weather around March 13.  Beyond that, temperatures through March 20 are likely to average below normal with above normal precipitation slightly favored.

Although a colder, wetter  than normal end of winter probably may sound bleak, consider average daytime temperatures by March 20 climb to 57 with lows averaging in the upper 30s.  Temperatures are forecast to average less than 5 degrees below normal.   That opens the door for plenty of days with highs in the 50s.  Plus, the really cold shots only last a day as the sun quickly moderates arctic air masses.  If there’s any snow, accumulation on roads is probably limited to the night time hours as the sun’s radiation heats them up.

Below is a plot of forecasts for the temperatures at Dulles airport for the next 7 days from a set of simulations (ensemble) from the GFS model (GEFS).   Most predict next Monday, Tuesday and Wednesday to be in well up into the 50s if not into the 60s.


Plot of ensemble member forecasts of temperatures at Dulles Airport. Each line represents a model forecast using the same model but with slightly different initial conditions. The orange line is the median of all forecast and the black line is the average of all members. The start of each day is in GMT so 8 MAR is actually 7 p.m. on March 7.

However, by Wednesday the forecast gets tricky as almost all the simulations have some type of storm approaching us giving us a higher than normal probability of getting precipitation.  Most members suggest the precipitation would fall as rain (see below) but several  of the simulations track a low to our south which might draw enough cold air into the region for  precipitation to end as snow.  One simulation even forecasts quite a bit of it.  Right now the snowy simulations are outliers but the storm still is worth monitoring as forecasts that far out in time can sometimes  change significantly as the storm approaches.


Plot of ensemble member forecasts of precipitation and precipitation type at Dulles Airport. Each line represents a model forecast using the same model but with slightly different initial conditions . When the lines are climbing precipitation is falling, the steeper the incline the heavier the precipitation. When lines are flat no precipitation is falling. The start of each day is in GMT so 8 MAR is actually 7 p.m. on March 7.

Why do you still expect temperatures to average below normal from March 11-March 20?

The upper air pattern that has dominated our weather this winter shows no signs of breaking through March 20.   Most of the winter the jet stream has been displaced northward into Alaska or western Canada as a strong upper level ridge has been locked over that region.   Times when Alaska and western Canada warm up,  the East usually ends up being cold.   The models are suggesting that pattern will hold.

What’s the outlook for March 11 through 16?

The European model ensemble (its grouping of simulations) continues to show a positive Pacific North American (PNA) pattern featuring a strong ridge of high pressure  in the West and a trough of low pressure in the East in the March 11 to 16 period.  The  high altitude winds (at 18,000 feet or 500 mb) flowing parallel to the black lines on the figure below signifies weather systems tracking from Canada southeastward into the eastern U.S.  That almost guarantees more colder than normal days than warmer ones.


High altitude weather pattern for period Mar 11 to 16 from European model ensemble (WeatherBell.com)

(Note: the ensemble forecast here is a 5-day average (or mean) of the high altitude pattern because the larger scale, lower frequency waves in the atmosphere stand out on those maps and that type of wave is much easier to predict than the faster moving smaller scale waves.   Plus, these larger scale waves tend to help steer the smaller scale waves that often are associated with our storms.)

The European model ensemble temperature differences from normal at 5,000 feet (850 mb) for the same period are shown below.  The pattern of having above normal temperatures across the western North America and below normal  temperatures across the East is a classic positive PNA pattern.  At around 5000 ft our temperatures average around  2.5oC or 4.5oF  below normal , which is chilly but hardly frigid.  That doesn’t preclude a really cold day sneaking in but suggests any really cold day will be balanced by a day near or a little above normal.  It also suggests our cold days will probably have highs in the 40s to low 50s  and lows in the upper 20s to low 30s.


(WeatherBell.com)

The storm expected to affect the region around March 12 should keep the precipitation for the 5 days above normal.  Could it give us snow? Yes – perhaps there’s a little as the storm pulls away. But to me accumulating snow is a long shot.  A mostly rain event seems more likely.  Still as I noted earlier, the storm is worth watching over the next coulple of days.

What’s the outlook for March 15-20?

The pattern shown on European ensemble forecast for this 5-day period (see below) has a more complicated look than on the figures just discussed .   There is still some ridging near the North Pole and a trough (blue shades) across the East.  The latter signifies that the jet stream will be displaced southward from its usual position for this time of year.  That usually will lead to below normal temperatures.  However,  the strong ridging along the West Coast has given way to a more zonal look.  The height (or pressure) lines across the Pacific into West run from west to east.   That argues that Pacific systems may start traversing the country.  Such systems are not a cold as those with more northern origins.   However, the flatter look also could be from differences of timing of systems and the natural damping of amplitude of features in the ensemble mean as you go out in time.   If indeed Pacific systems cut across the U.S., the pattern would probably end up being wetter than normal.


(WeatherBell.com)

The ensemble mean temperatures around 5000 ft (850 mb) from the European model for this period still indicated cooler than normal conditions are likely to prevail across the East (see below) but only a few degrees below normal.  A huge caveat is needed.  At such long time ranges,  cold and warm signals are often muted as it is hard for the various simulations (ensemble members) to come to a consensus.  Plus,  daytime surface temperatures sometimes can be quite a bit colder than  temperature forecast at 5,000 feet (850 mb) would imply in cases of cool air damming with easterly flow.  Such days can really mess up temperature outlooks.


(WeatherBell.com)

The bottom line is the period looks cooler than normal but hardly looks frigid but I’m not confident about that call.  Anytime the trough is in the East there is the potential for storminess.  Overall, the March 15-20 period pattern looks complicated with plenty of uncertainty.

Conclusions

After our warm-up early next week, cooler than normal weather is favored to return.  While there is still the potential for another very cold day or two in the lead-up to spring, the intensity and duration  of the cold in this upcoming period will not match what we’ve experienced through early March and more and more days in the 50s will enter the mix.  There is no high confidence snow threat during this period, but we need to watch the storm around March 12 just in case.

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Jason Samenow · March 7