Major east coast storm possible later this week
The models are finally beginning to come to a better consensus on the ideas that I had posted in the comments section the last couple of nights, so I thought it was time to provide a more in depth discussion of our potential storm system.
The pattern over the Pacific has changed over the past few days with the trough that was located over eastern Asia translating into the Pacific and beating down the persistent central Pacific ridge. We went from this (note the trough over eastern Asia, ridge over central Pacific, trough diving into the west coast):
to this (trough diving into Pacific, eastern Pacific ridge):
This will began to have significant implications downstream, as the ridge will rebuild along the west coast. This will in turn cause plentiful energy coming across the Pacific to round the ridge and carve out a trough over the country’s mid-section. This trough is then expected to turn negative and ride up the eastern seaboard.
Take a look at the water vapor satellite. Note all of the jet energy approaching the west coast (yellow and red shading). This is a big signal that a powerful storm is in the making. All of this energy will be helping to carve out a deep trough:
By Wednesday night this turns into:
The models remain at odds on the track of the storm, but a general consensus would argue for rain changing to a heavy wet snow as dynamic cooling takes effect with very cold air aloft moving overhead.
Above we have this morning’s GFS run for Friday. Note the negatively tilted 500mb trough, and the northern and southern jet streams are in the process of phasing. The 700mb low is beginning to intensify and moisture is getting thrown back. This moisture is being dynamically lifted, shown by the positive advection of vorticity. At this prog, there is a sub-988mb low off the coast, and the boundary layer is cooling enough for precipitation to changeover to snow.
The one thing that I think is worth mentioning is the fact that you can see on the GFS image above how there is no high on the backend filtering in and thus a lack of low level cold air on the backside of the storm. We will be relying mostly on dynamic and evaporate cooling to allow the column to cool. Right now I think the best chance for an accumulating snowfall on the backend of this system would be over the interior areas north and west of NYC. But, the city and coast need to keep a close eye on the progress of this storm. Coincidentally, since there is no low level cold air filtering in behind this system, it will actually get warmer this weekend after this storm departs and heights rise. So whatever snow does fall will be rather quick to melt in the days following.
Updated: 7:36 PM GMT on March 29, 2011
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Negative NAO returns with a vengeance and sends us back into winter
Short Range Weekend Discussion (updated Thu 3/24)
This Weekend’s Blocked Pattern:
The energy that is blocked between ridges over western Canada and the eastern North Atlantic/Greenland will continue to consolidate into a powerful upper low that will gradually sink southward. This “wedged in” polar vortex will crush down heights over the northeast. Take a look at our temporarily blocked up pattern below. A strong upper high building back toward Greenland and a ridge over western Canada with a polar vortex over the Canadian Maritimes:
850mb temperatures have dropped into the teens today and are not expected to get out of the teens until next Tuesday! The graphic below shows high pressure building down from Canada and northwest flow sending down unseasonably cold air straight into our region:
The dotted blue line on the graphic above is the -10°C, or 14°F isotherm at 850mb, and you can see how that isotherm has sunk to the south of us by early Saturday. You can also see that moisture is getting suppressed to the south. Even with full sun (now at a mid-September angle) and excellent mixing with deep-layered northwest flow, these anomalously cold 850mb temperatures of 10 to 15 degrees will only be capable of translating into highs near 40 on Friday and in the upper 30s on Saturday, with breezy conditions generally keeping wind chill values from getting much higher than the freezing mark through both days. Lows at night will fall deep into the 20s area-wide, with winds dying down enough for teens in the far outlying areas. This will almost certainly be the coldest air mass we experience until next winter.
The NAM is likely too cold (it shows 850mb temperatures dropping into the single digits this weekend!), but I’m posting it here anyway for illustrative purposes. Look at all of the cold air funneling down over the northeastern quarter of the country ahead of a 1035mb high over south central Canada, with a strong thermal gradient set up to our south:
On Sunday, it looks like northern stream energy diving down from southern Canada into the Great Lakes will be strong enough to keep Pacific moisture suppressed just to our south:
If precipitation were to fall, it would be cold enough for light snow. The best chance of this occurring would be over southern portions of the metro area. However, I personally believe that the polar energy diving down from Canada will be strong enough to keep us dry. Temperatures on Sunday will be somewhat dependent on how much cloud cover can make it into the region, with highs somewhere in the upper 30s to lower 40s expected. This same energy sends one more push of colder air into the region on Monday with highs once again capped in the low 40s.
So to recap, a cold late March weekend is on tap from Friday right on through Monday. The dark blue shaded area (which includes NYC) on the ECMWF ensemble mean image below shows areas that are expected to encounter 850mb temperatures averaging greater than 8 degrees below normal this weekend:
Surface temperatures are expected to be averaging some 10 to nearly 20 degrees below normal from east to west across the northeast:
Stay warm and have a great weekend
Quick shot of snow expected tonight but little accumulation
Near Term Discussion (updated 3 pm on Wed 3/23)
The rain/snow line has been hovering in our area during the day today with periods of rain and wet snow at times. It gets a little more interesting as we head into tonight, and let me show you why. As we head into tonight, our 500mb shortwave will be approaching. Right now what we are dealing with is basically overrunning because the flattened 500mb shortwave ridge axis won’t even be clearing us until early this evening. Once it does so, there will be a relatively small period of time during which there will still be ample mid-level moisture AND forcing at 500mb to act on this moisture.
Forcing via positive advection of vorticity (energy)…
+ High levels of moisture in the mid-levels…
= Moderate precipitation…
This period of time will be from late this evening through the first part of tonight. Now the other piece to this equation is that coincidentally, temperatures are expected be dropping to near or just above the freezing mark before midnight.
In the form of snow…
So this leaves us with a short stretch of time where moderate precipitation is expected to fall in the form of snow. But still, as I was saying last night, the ground will be quite warm and wet, so it will be very difficult to actually get the snow to stick, especially in the city. The suburbs may have a slightly better shot.
Short Range Discussion (updated Tue 3/22): Overview of the pattern leading up this week’s storm
While the exact track of the storm system for later this week is still uncertain, I’d like to take a quick look at some of the features that will be setting up for our storm. Here we have tonight’s 500mb pattern analysis as depicted by the NAM:
Let’s begin analyzing this map from the top right hand side. First, we have the high pressure ridge that is beginning to build up over the eastern north Atlantic (and will gradually retrograde northwestward back toward Greenland over time). This is in association with the falling NAO index (which I discussed in more detail back in my last entry). This ridge is causing everything to get blocked up. Rather than the disturbances escaping to the east like they were for the past weeks, now they can’t because the upper level high pressure ridge is preventing them from doing so. Disturbance #1 is remnant energy that is left over of the upper level disturbance that sent a dry cold front through last Friday and brought an abrupt end to the 70+ degree weather. Disturbance #2 is the upper level energy that was responsible for the rain and snow Tuesday morning. Disturbance #3 is a perturbation in the polar jet stream. These three disturbances will not be able to escape to the east due to the downstream blocking ridge.
Now, what makes this even more interesting is that there will be a quasi-stationary upper ridge over western Canada, ever so slowly changing tilt on its northern end. Strong northwest flow on the eastern flank of this ridge will send down additional energy. Together, all of this trapped upper level energy will ultimately form a powerful upper low.
In the meantime, disturbance #4 is a Pacific disturbance that will be crossing the country around the southeast high pressure ridge. In the previous pattern, this disturbance would have had no trouble moving to the north of our area like the past ones. However, once an upper low begins to form to our north, disturbance #4 will be forced to the south by energy and northwest flow rounding the vortex. This will cause the disturbance to make a sharp turn to the east and then southeast.
Right now there are small differences in the details that are leading to very different solutions. The models will continue to iron themselves out on the exact track of this disturbance and the associated low pressure system over the next few runs. Check back for updates.
Updated: 7:35 PM GMT on March 23, 2011
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Pleasant weekend followed by a brief return of blocking next week
Short and Medium Range Discussion (updated Sat 3/19)
Warm 850mb temperatures and excellent mixing this afternoon combined with a downsloping wind component and sent temperatures soaring through the 70s. The record was crashed at Kennedy with a high of 79, breaking the old record of 70 by 9 degrees. Temperatures hit the 80 degree mark at Newark. While a southeast ridge will remain strong right through early next week, here in the northern mid-Atlantic we will be moving onto the polar side of the jet with temperatures on their way down as a moisture-starved shortwave moves on through Saturday.
Ahead of this upper level wave, a surface cold front crossed through Friday, with 850mb temperatures crashing Friday night. In fact, by late Saturday, 850mb temperatures will be as far below freezing as they were above freezing Friday. The atmosphere will remain well-mixed during the day Saturday, so highs will still manage to reach the 50 degree mark one more time. The same cannot be said for Sunday, however, with high pressure building overhead severely limiting mixing, but good solar radiation from stronger sunshine this time of year still sending us into the 40s. Take a look at the strong 1036+ mb high overhead on Sunday:
As for the nighttime lows, with the cold air in place, temperatures Saturday night will drop into the 30s, with 20s in many suburbs. With light winds expected Sunday night, lows will depend on just how quickly clouds filter in overhead, with eastern areas having a better shot at a longer period of radiational cooling.
In the meantime, a disturbance will eject itself from the upper low near the west coast and slide to our north on Monday. Not only will the forcing from this shortwave be focused well to the north, but with the southeast ridge still pumping strong, there won’t be any opportunity to tap into Gulf moisture, so we probably get away with just some warm advection showers.
However, more importantly is the fact that this disturbance will combine with energy from the polar jet stream and be one of the building blocks of an upper low that is expected to form over the Canadian Maritimes. Isentropic lifting over the cold air mass that is deposited could even lead to some light snow Wednesday night somewhere in the northeast or mid-Atlantic depending on the exact track of the west coast low that will be finally ejecting eastward by the middle of next week.
Take a look at the graph below:
Both the GFS and ECMWF operational and ensemble mean are showing a –NAO forming by the middle of next week. This would end up being the first time we’ve had a substantial negative NAO since way back in January. This –NAO would signal a ridge building in the north Atlantic back toward Greenland. This will keep the disturbances from escaping to the east and lead to sneaky troughing in the northeast despite an unfavorable pattern as teleconnected downstream from the Pacific. Furthermore, additional ridging will be progressing over northern Canada and may “meet up” with the Greenland ridge to produce a high pressure block and cause a second, larger and more powerful, upper low to form underneath over northern New England/the Canadian Maritimes.
You may have noticed that the AO index is forecast to be quite positive over the next couple weeks, but this won’t matter too much because as we can also see on the image above, the +AO is due to the fact that there is a strong polar vortex centered over the arctic but more focused over the eastern hemisphere.
The negative NAO and blocking do begin to disappear after next weekend, but coincidentally some big changes may be going on the Pacific that could lead to another opportunity for some colder air in the extended range. I’ll keep watching the maps this weekend for more evidence on that potential change and post on it sometime soon.
Enjoy the weekend
Updated: 6:13 AM GMT on March 19, 2011
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Canadian air mass cools us down but with some difficulty
Short and Medium Range Forecast (updated Sun 3/13)
An upper level disturbance will scoot by to our north on Sunday. This disturbance is currently over the Great Lakes, depicted as disturbance #1 on tonight’s 500mb pattern analysis by the RUC model:
The surface cold front in association with this disturbance will pass through tonight, and winds will begin to veer. In addition, temperatures at the 850mb level will be dropping from the upper 30s this evening to around 20 by Sunday morning. This would often be indicative of a significant cool down; however, a strong downsloping westerly flow and good mixing will allow temperatures to rise up to near 50. But there will be a sharper drop in temperature observed Sunday evening as Canadian high pressure builds in and the effects of the new colder air mass begin to be felt. Lows will fall into the low to mid 30s.
Another shortwave (disturbance #2 above, currently over south central Canada) will approach on Monday, and this will lead through a mid-level trough that will send down some even colder 850mb temperatures in the mid-teens.
Mid-level temperatures this cold have often kept us struggling to reach the freezing mark with subsidence associated with high pressure building in. However, this time around, the alignment of the winds in the 850mb-surface level that will be fostered by this second shortwave passing overhead will allow us to still see highs climb well into the 40s with winds lined up out of the north in this level and good mixing once again despite the strong high building in.
The high will move overhead early on Tuesday and then offshore by late Tuesday. Clear skies and diminishing winds should allow for decent radiational cooling conditions Tuesday night with lows dropping into 30s in the city and 20s in the outlying suburbs. An onshore flow is expected on Tuesday with highs ranging from the low 40s near the coast to the upper 40s inland.
Shortwave ridging will crest late Tuesday before moisture begins to stream into the area ahead of a southern branch disturbance. That disturbance will be meandering in the southeast until getting picked up by a northern stream trough. Before it does so, a round of showers will move through the area during the day on Wednesday.
The consolidated trough passes overhead late Wednesday night. Behind this trough, ridging will build with milder weather expected for the end of the week. A powerful trough will be dropping into the west coast, and this will pump up the southeast ridge and allow the northern end of its influence to extend into our area. Look at all the milder air that will be streaming in:
This will send us well into the 60s for highs by Friday, followed by more normal temperatures over next weekend.
Updated: 3:44 PM GMT on March 13, 2011
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