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Posted at 12:30 PM ET, 01/14/2011

Will early Tuesday be wintry or rainy?

By Wes Junker

Subtle pattern changes have taken place with implications for the next storm system that will approach the area late Monday night into Tuesday. These changes almost guarantee that the next storm will be primarily be a rain event for most of the area, especially east of I-95.

However, enough residual cold air may remain across the region to allow for a brief period of snow, sleet and/or freezing rain when the precipitation begins early Tuesday morning, particularly west of the city. The risk of wintry weather is highest toward the Shenandoah Valley where the cold air often sticks around longer than near the city.


The main change in the pattern is that the North Atlantic Oscillation (NAO) has changed signs from negative to positive as shown below. In the past I've discussed the NAO and its impact on our weather and how the negative phase increases our chances of cold weather and snow.

North Atlantic Oscillation values since late September (black line) and forecasts for the next couple of weeks (red lines).

The change from a negative to a positive NAO signals less blocking at high latitudes which will make it easier for the next storm track to the Great Lakes region and harder for cold air to sit over the northeast prior to any storm, supplying cold air.

A more in-depth look at the pressure pattern that most of the models are predicting might shed some light on why it does not favor snow. The biggest problem with the pattern if you want snow is the surface pressure pattern across the Great Lakes region into New England is in the exact opposite configuration that's favorable for snow.

Last night's (0z) 102 hr GFS simulation of mean sea level pressure, 2 meter temperatures, winds and precipitation valid at 7 a.m. Tuesday, Jan. 18. The black arrows illustrate the wind direction.

One of the first forecast rules I learned concerning winter weather was that for snow and cold air damming it is better to have the pressure higher over the northern Great Lakes than over northern New England. The reason is that air moves from higher to lower pressure so a high over the Great Lakes promotes cold low level flow from the northwest into our area if there is also low pressure near Nova Scotia. In Tuesday's case, the models are showing the opposite pressure configuration (lower pressure over the Great Lakes, and higher pressure over Nova Scotia) as shown above. A big strike against snow.

Now note that high pressure has pushed off the New England coast and is located just south of Nova Scotia. The flow around a high pressure system is clockwise which means the winds across the Northeast and mid-Atlantic are either from the south or southeast originating over the ocean, which at this time of year is a repository for heat. The position of the low over the Great Lakes produces winds from the south on its east side - in our direction.

Both the forecast positions of the high off the coast and the low over the Great Lakes prevents cold air from coming south and instead promotes warm air streaming northward. Winds from the southeast extend from just off the Virginia coast to the eastern Great Lakes.

The GFS model does simulate some stale cold air lingering early Tuesday morning, with the freezing line hovering near the District and modestly colder air to the west. However, with this type of pattern the models sometimes trend warmer with time.

Taken literally, last night's GFS and the run this morning would include a period of sleet or freezing rain at the event's onset that could last long enough to cause glazing west of the city. It's also not out of the question the precipitation could begin as snow but not likely.

Why am I so confident that this will primarily be a rain storm with a chance for some freezing rain or sleet at onset? The situation is different from those earlier cases where we provided a number of possible scenarios for the storm. They still exist but all the different scenarios track the primary low to the Great Lakes region and all take the high pressure system off the coast. The ensemble members from both the GFS and European model are pretty emphatic about the primary going to the Great Lakes (see this link for the GFS ensembles from last night's run valid Monday night and note the position of the low). Remember that GFS ensembles are the same model run at lower resolution with slightly different initial conditions. Note the similar looks of all the members and that quite a few are a little warmer than the operational.

In summary, this morning's GFS continues to support that the upcoming storm will probably primarily be rain except for a few hours at the onset. The biggest question mark about this system is how long will the cold air hangs on across and especially west of the city. If it lingers, it could last long enough to cause a glaze that could cause some slick spots late Monday night into early Tuesday morning especially to our west. That would obviously have implications for travel, schools, etc. The Capital Weather Gang will be monitoring this situation.

By Wes Junker  | January 14, 2011; 12:30 PM ET
Categories:  Latest, Winter Storms  
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Next: PM Update: More cold, remaining mostly dry


Yeah, rain! Good for lawns, gardens, and washing the salt licks off the sidewalks and roadways.

Posted by: natsncats | January 14, 2011 12:34 PM | Report abuse

booo! rain... ;-(

Posted by: walter-in-fallschurch | January 14, 2011 1:41 PM | Report abuse

As I posted earlier the worst feature of this storm could involve cold rain Tuesday evening while I'm trying to get to my dance.

Unlike TominMichiganParkDC, I wouldn't mind it if the rain ended about sunset Tuesday...this would make it two dance Tuesdays in a row so far this year impacted by bad, nasty weather.

Posted by: Bombo47jea | January 14, 2011 2:08 PM | Report abuse

Wes, I agree that the next system will be primarily a rain event. However, I think there's a good chance the cold air near the surface (cold air damning) will hang in longer than indicated by the GFS and, hence, a longer period of mixed precip, including freezing rain.

In my experience, at least, the GFS systematically gives way to the influx of warmer air into the coastal plain sooner than predicted - if for no other reason it (GFS) does not have sufficient resolution to resolve the terrain (and its influence) between here and the Blue Ridge/Appalachians to the west.

Posted by: SteveT-CapitalWeatherGang | January 14, 2011 2:10 PM | Report abuse

It appears to me that only in the last week or so has the basic pattern associated with La Nina (beyond the tropics) kicked in.

The same was true last year in regard to El Nino. The pattern change didn't occur until January.

Posted by: SteveT-CapitalWeatherGang | January 14, 2011 2:16 PM | Report abuse

wes, steve,
when i look at the GFS loop, i see the cold coming back towards the end of the storm:


any chance this ENDS as snow?

Posted by: walter-in-fallschurch | January 14, 2011 2:17 PM | Report abuse

Steve, I think that's true if the high is in a decent place (over NEw england) but when it's offshore and the low level winds become easterly I'm not sure that's true. if we have a stronger coastal trough then sure. I guess we'll see.

Posted by: wjunker | January 14, 2011 2:23 PM | Report abuse

Posted by: wjunker | January 14, 2011 2:32 PM | Report abuse

Tuesday morning is a long way away.
Nevertheless I am now officially on orange alert for that morning's commute.
I'll take that rain though. Hope it rains all day long. Wash out the salt; hydrate the parched landscape.

Posted by: FIREDRAGON47 | January 14, 2011 2:33 PM | Report abuse

walter-in-fallschurch, the gfs does indeed have some snow at the end of the event. The upper impulse associated with it looks pretty weak so I'm not sure what to make of it. The GFS does sometimes overdo the light precipitation in the cold air. We'll be watching it over the next few days.

Posted by: wjunker | January 14, 2011 2:48 PM | Report abuse

It seems like the models are trending somewhat colder with time.

While it doesn't look like a major snow event (maybe an inch or so at onset), there could be a significant ice storm from DC west.

There appears to be just enough of a coastal development to keep the low level cold air in place.

Posted by: frontieradjust | January 14, 2011 6:35 PM | Report abuse

While I am as giddy as Walter over snowstorms, the lack of ANY precipitation this winter is even more frustrating.

So, I will take all the rain we can muster: even if it has to be cold, raw, dank, chilled-to-the-bone rain.

Anything to hydrate the landscape, clear the air, and wash the streets clean.

Next up: a count of the number of Tuesdays that precipitation actually fell at National Airport last year! Threatening skies do not count, of course...

Posted by: TominMichiganParkDC | January 14, 2011 7:50 PM | Report abuse

Rain, please. Thank you. Save 10 bucks at the car wash.

Posted by: moo1 | January 14, 2011 8:33 PM | Report abuse

snow is good for plants, right? insulation. slow release of moisture.

Posted by: walter-in-fallschurch | January 14, 2011 8:43 PM | Report abuse

Justin Burke from WMAR TV in Baltimore stated he feels the models are overdoing the warm air creeping down to the surface for the Baltimore region and that a prolonged period of icing will take place. Any chance this could actually happen?

Posted by: jcmcgrath1969 | January 14, 2011 8:46 PM | Report abuse

Justin Burke from WMAR TV in Baltimore stated he feels the models are overdoing the warm air creeping down to the surface for the Baltimore region and that a prolonged period of icing will take place. Any chance this could actually happen?

Posted by: jcmcgrath1969 | January 14, 2011 8:47 PM | Report abuse

NWS says light precip Tues. We really need more than that!

Posted by: moo1 | January 14, 2011 11:35 PM | Report abuse

just got back from my snow trip down to SC,,tHEY HAD A MESS DOWN THERE, but it was fun.

Ice storms IMHO should never be taken lightly. I have seen first hand what they can do down south. if its not gonna be snow. I will take rain over ice anyday.

Still hoping for that decent snow( 5-8 inch) before winters over up here.

Posted by: BradFinWoodbridgeVA | January 14, 2011 11:36 PM | Report abuse

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