Warm ridge over east, followed by severe weather?
Potential for severe weather on Wednesday?
There are numerous forecast ingredients indicating the possibility of severe weather in parts of the mid-Atlantic states on Wednesday. As such, the SPC has already highlighted this possible threat in their D5 outlook.
I want to briefly outline some of the projected indices. Keeping in mind that these numbers outputted by the GFS certainly cannot be taken as verbatim this far out, here is the current impressive MLCAPE forecast for Wednesday afternoon.
The GFS shows mean layer and surface based CAPEs of 2000-4000+ J/kg Wednesday afternoon along the I-95 corridor. While this may not verify as printed, it does give us a sense of just how great the thermodynamics will likely be on Wednesday that could provide the necessary energy for severe thunderstorm development. There will be plenty of mid-level dry air to contrast with the warm, moist low levels (dewpoints in the upper 60s).
The mechanism for initiating convection will be the approaching cold front. Lifted indices are currently forecast by the GFS to drop below -10 over a widespread area, which is indicative of a very highly unstable environment.
Wind shear will also increase as the jet stream becomes closer in proximity. The GFS is showing 30+ kts of 0-6km bulk shear in the majority the mid-Atlantic region by Wednesday evening.
This may end up being the limiting factor. Speed and directional wind shear are both somewhat lacking. I would like to see greater speed shear values with such high CAPE for organized severe thunderstorm development.
(Continue reading for the weekly forecast discussion)
Impressive 594dm ridge over east early next week
As a deep trough digs into the western US this weekend and with a lack of high latitude blocking, model guidance is in good agreement on a ridge surging up in the eastern US and building through early next week. GFS ensembles are suggesting that much of the mid-Atlantic region will be kicking off the unofficial start of the summer season with very impressive 500mb heights of 594dm.
On Memorial Day, we will have a 10-15 mph boundary layer WSW flow with fairly good mixing and downsloping east of the Appalachians will further enhance temperature potential under this wind configuration. The white shading on the image below represents 850mb temperatures forecasted by the GFS to be greater than +18C, or 64F. Under these conditions coupled with decent mixing and downsloping, many areas near the I-95 corridor will likely make it into the low to possibly mid 90s. Heat indices will be in the upper 90s.
With the ridge axis just to the west of the Appalachians, one thing that we will have to be wary of is the possibility of a weak backdoor cold front sinking to the south Monday night. This would act to suppress the core of the heat on Tuesday. 850mb temperatures would fall slightly and winds would veer and introduce a greater easterly component to the surface flow, thus losing the downsloping effect and allowing for greater marine influence along the coast. The NAM, ECMWF, and SREF ensembles are in good agreement on this front, with the GFS being more of an outlier in washing it out. The higher resolution of the NAM and ECMWF may be better able to capture this weak surface feature that the GFS loses more quickly.
The ridge will retrograde mid next week as a trough and associated cold front roll on down with thunderstorms on Wednesday and cooler temperatures for the end of the week with falling heights.
Temperatures on Friday will only make it into the low 70s along with low humidity. Overnight lows will be in the 50s area-wide.
Looking further in the extended range, a retrograding ridge over the north Atlantic could bring back a negative NAO pattern and act to keep troughing over the northeast, with the northern mid-Atlantic region under northwest flow aloft and sunshine likely allowing for near normal temperatures and pleasant conditions. Below on the left is the ECMWF 8-10 day average height pattern and on the right is the GFS predicted height pattern for the middle of the following week, each featuring a negative NAO pattern.
The ensembles are indicating a falling NAO index heading towards next weekend, possibly turning negative at the end of the forecast period.
Have a great holiday weekend
Updated: 10:55 PM GMT on May 28, 2011
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Unsettled weather conditions continue but for how long?
Just to quickly follow up on my last discussion, the 90s will not be happening this week as our energy is expected to weaken considerably and close off, thereby sealing off the possibility of an omega block developing. However, it will still warm considerably on Tuesday before a front pushes to our south and cools us down. The forecast becomes more interesting in terms of what happens as we unofficially kick off summer on Memorial Day and the days immediately following, with substantial disagreement once again amongst the models, which in fact has been the case for the last several days and continues with the latest model runs. Each result would have a big impact on temperature and precipitation.
So we begin with this week. Our trough will slowly continue to depart and ridging will build through the day on Sunday. Ordinarily this would result in even warmer weather conditions than Saturday; however, a backdoor cold front continues to sink to the south, and this will lead to a cooler air mass settling in and thus cooler temperatures along with plenty of clouds. A similar situation will be on tap for Monday as well, though it will be milder with a greater southerly component surface air flow. Warm advection showers will be possible Sunday night. Then, a weak mid-level shortwave and surface trough will move overhead late Monday into Monday night with showers and possible elevated thunderstorms.
A cold front will approach on Tuesday, and out ahead of the front, winds will veer around to the west and all of the ingredients will be in place for temperatures to be off to the races. 850mb temperatures will be in the upper 50s to near 60:
This warm air mass overhead along with plenty of sunshine coupled with aligned west winds and downsloping will allow for temperatures to soar into the upper 80s. Under this wind configuration, the warmth will spread right up to the coast. One thing that we may need to keep an eye on is the potential for isolated strong thunderstorms to develop. With not much shear but moderate surface and mixed layer CAPE to work with after daytime heating, we may see some pulse storms pop up with a lack of organization:
A shortwave will roll through on Wednesday and likely spark more shower activity, and additional energy riding in later in the week may ignite more showers or thunderstorms as we remain in close vicinity to a stationary front:
Exactly where the front sets up shop will be important in determining temperatures during the period. The ECMWF model sends the front back northward sooner than the GFS and is warmer overall (largely due to the differences in handling of the slowly departing trough discussed last week, with the EC closing it off again and allowing ridging to build over top and connect us with the Atlantic ridge, while the GFS keeps it open). Regardless, expect generally above normal temperatures and more of the same heading into the first part of the Memorial Day weekend. That’s where it starts to get more interesting.
The GFS and ECMWF models are at it again, each featuring starkly different solutions in the extended range:
The ensemble mean of each of the two models (not shown) offers a more moderate solution, but it is basically a compromise between the two possible outcomes. It looks to me like it is more than likely one or the other. The key feature is a strong piece of energy that will be coming ashore and scooting along the US/Canadian border. It is what happens then that varies between the two models. The two different potential paths of this feature are shown below, with the GFS model on the left and the ECMWF on the right:
The GFS is showing this energy getting picked up by a shortwave in the powerful northern stream trough rotating around over eastern Canada. In the meantime, a strong upper low will be dumping into the west coast. The aforementioned energy rides around the building ridge being pumped up by the west coast trough, and temperatures over the entire eastern half of the nation soar through much of the week as a 594dm upper high builds over the midwest. Furthermore, the energy that rode around the ridge drops into a weakness still off the coast over the western Atlantic and an omega block forms. So, we experience an extended period of dry sunny weather and maybe a heat wave with temperatures possibly surpassing 90 for three days. Here is what the GFS shows by next Tuesday. Note the large swath of 20+ degrees Celsius 850mb temperatures (20 is the red line on the bottom right panel), which would include the NYC area:
However, don’t get your swim trunks ready just yet, as the ECMWF model couldn’t be more different. It believes that the energy does not get picked up by the northern stream. Rather, it slides down the eastern flank of the building ridge ahead of the now slowed low approaching the west coast and into an eastern US trough. The energy forms a 546dm southern stream closed low in the same general vicinity of where the 594dm high is at the same time prog on the GFS:
You can see in the image above how regardless there is likely a very amplified upper level pattern, but the question is in which part of the block do we end up, which will have a significant impact on sensible weather. In addition to its significant effect on temperatures, the ECMWF solution shows a strengthening low level jet pulling up Atlantic moisture and making for a cool, damp end to the Memorial Day weekend, followed by another slowly weakening and departing low in the first few unofficial days of summer, likely leading to more unsettled weather:
The reason why I don’t think a middle of the road, flatter solution is likely is that the whole large scale pattern is dependent on the fate of this energy. If the energy is picked up, then I feel that the powerful upper low will have no trouble sinking into the west coast and thus a strong ridge pumping up over the east. The few GEFS ensemble members showing a zonal flow just doesn’t seem right to me given that strong upper low coming in from the Gulf of Alaska. I think we are in for a very amplified flow pattern over North America in the extended range, but once again, the main question that remains is where will everything fall into place? Since writing this, the 18z and 0z runs of the GFS have begun to edge slightly closer toward the ECMWF solution, showing the energy "missing" the northern stream and forming a (weaker) trough over the east, followed by Bermuda high pressure keeping us in a damp maritime tropical air mass:
It will be interesting to follow what actually transpires.
Updated: 5:31 AM GMT on May 22, 2011
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Examining ECMWF rumors of 90 degrees next week
Tue 5/17 Discussion—What are these rumors of 90 degrees late next week?
There is little doubt about the idea of a warm up next week; however, just how long the warmth sticks around and whether it builds or recedes later next week has been in great debate amongst the models. Last night’s 0z suite outputted a wide variety of solutions, indicating that the start of the Memorial Day weekend could either be a little chilly this year with highs in the 60s and lows in the 40s (GGEM), versus remarkably hot and humid with highs near 90, heat indices likely approaching 100 and lows at night struggling to make it down to 70 degrees (ECMWF). The GFS solution falls somewhere in between.
Hot and Humid Solution:
As you can see above, 850mb temperatures at the exact same time prog of next Thursday evening range between 0°C and 20°C. This would make a world of difference. The new European model that came out this afternoon has taken a significant step in the direction of the GFS and is now showing highs in the 70s. The really interesting part is that a major feature in determining how this plays out is the upper low that is currently producing our soaker this week. Last night’s EC wanted to keep a more amplified trough over the western Atlantic in response to leftover energy from the low, and that further enhanced any ridging that was able to pop up over the east and kept it in place longer. This solution would be viable, especially if you consider how the energy from these upper lows is often slower to dissipate than expected. The GGEM on the other hand used the positive PNA ridging to dig a deep trough down into the eastern US. The whole look to this seemed a little extreme in my opinion though, as the GGEM often is. Right now I would argue for a solution somewhere in the middle, but I wouldn’t completely discount a warmer solution just yet. The new 12z EC is showing the energy from the upper low close off once again and slide underneath the western Atlantic ridge. This is producing a cooler solution than 0z.
Possible Solutions (from warmest to coolest):
Sat 5/14 Discussion—Blocked upper low to bring unsettled weather through next week
Take a look at this morning’s upper level pattern, which includes a huge western North America ridge, which can often be one of the precursors to the development of a closed low downstream (which is exactly what we are expecting to reform next week):
A very interesting set up will be comprised this week in the upper atmosphere. Today we have an upper low over the midwest which will be slowly translating eastward into the Ohio Valley through early next week. You can see the swirl of the low, as well as the large amount of moisture getting pulled up, on today’s water vapor loop:
This upper low will be gradually filling as it loses its punch (if you watch closely near the center of the swirl on the water vapor loop above, you can even see it starting to gradually to disorganize a bit towards the end of the loop) . However, additional energy rounding the western North America ridge will be dropping in from the polar jet stream and act to reenergize the upper level system:
At that point, the upper low will spin over the Ohio and Tennessee River valleys during the mid-week period, followed by a shift northeastward into the mid-Atlantic region late in the week with gradual filling. The upper low will become locked in by the large ridge drifting from western to central North America and a polar jet receding way up into northeastern Canada. As a result, a rex block will form for a time mid-late next week as the flow buckles and high pressure in the upper levels of the atmosphere becomes situated over the Canadian Provinces, directly to the north of the upper low:
This will result in some unseasonably warm and sunny conditions over parts of south central Canada, but a persistent period of cloudy and rather wet conditions in our region. Granted it will not be raining all the time, but when you add it all together, it will amount to one soggy week.
Also note the immense high latitude blocking late next week with our low trapped underneath:
It looks like we will see showers/thunderstorms develop on Sunday as a couple of mid-level shortwaves move on through. Then, drier air begins to filter in at the mid-levels on Sunday night and will remain overhead at least through the day on Monday. There will still be plenty of moisture around in the low levels and especially at the surface, so fog will be possible Sunday night, especially near the coast. Monday is a little dicey because the GFS is showing the low levels clearing out a bit more than the NAM, which would allow for perhaps a couple breaks of sun and temperatures soaring into the mid 70s area-wide. However, the NAM keeps us under a thick cloud deck and holds temperatures in the low 60s at best. This would also mean about a ten degree difference in overnight lows Monday night, with either another humid, summerlike night and min temperatures of around 60 degrees under a southeasterly flow versus lows dropping to near or below 50 in a northeast flow. The exact placement of the front will be key in determining this. The differences would also affect precipitation because being to the north of the front would allow for a surface over which overrunning could occur, with a more substantial rainfall Monday night as opposed to a couple passing showers. For what it’s worth, right now the European and Canadian model guidance both fall more within the NAM camp, so the GFS may be too far northwest with the front. Still, I’m not convinced that surface high pressure over Canada would be strong enough to overcome the strong southerly mid-level air flow which will be attempting to keep the front well inland. We’ll just have to wait and see about this portion of the forecast.
A more significant shortwave rotating around the upper low will bring in a nice feed of moisture off the Atlantic and combine with a sharpening gradient between the surface low and high pressure over the Canadian Maritimes to overrun and soak us with a steady rain from Tuesday into Wednesday:
Drier air filters in overhead once again on Thursday, followed by what is left of the filling low passing overhead on Friday. The cold pool aloft with 500mb temps approaching -20°C will generate instability and lead to some showers developing during the day on Friday:
I am glad to be able say that improving weather conditions will likely greet us next weekend as the trough/low slowly pulls away (likely slower than the GFS is showing, probably more like the EC).
Updated: 11:35 PM GMT on May 17, 2011
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Omega block gives us a week of wonderful spring weather
Over the next several days, we will continue to experience what will end up being some of the nicest weather of the next few months. An upper low has formed off the coast largely from the bundling energy resulting out of the negative NAO pattern.
This low will continue to spin ever so slowly southeastward, and it will continue to keep a very pleasant, dry northerly flow across the area. The ridge axis currently located over the midwest will continue building into the region through Friday. As a result, rather than being entrenched in the progressive pattern that we were in earlier this spring, with system after system affecting us and wet conditions every few days, we will now be in a slow moving pattern—and in just the right location in the pattern at that for beautiful spring weather conditions. 850mb temperatures will be right around normal for this time of year in between the two weather features. The wind will pick up a bit on Tuesday as high pressure noses down slightly from eastern Canada and contrasts with low pressure off the coast associated with the upper low.
Continued low dewpoints being drilled down will mean comfortably cool nighttime lows in the upper 40s most spots to low 50s in the city, and mild daytime highs in the 60s east of the city (closer to upper low and lower heights) to low 70s west. Another interesting feature of this weather pattern is that with the strong northerly component boundary layer flow, we will not see any seabreeze develop at the coast, with mild temperatures continuing right into the evening without a quick drop-off often seen this time of year with the cold ocean temperatures. A weak shortwave rotating around the upper low will mean a slight increase in clouds on Wednesday, but no precipitation is expected. High pressure will be overhead late in the week with a decrease in winds but an increase in afternoon seabreeze development. Still no rain on the horizon until the weekend.
Updated: 11:59 PM GMT on May 09, 2011
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Negative AO/NAO pattern returns with more troughiness for northeast
A cold front will slowly push through the area, with showers expected behind the front during the day on Wednesday. Heights will fall through Wednesday night, squeezing 1000-500mb thicknesses to sub-534dm. This will lead to some rather chilly spring weather, with temperatures on Wednesday failing to get out of the 50s under cool cloud-covered northwest flow. Lows starting Wed night will be falling into the mid 40s in the city and upper 30s in some of the suburbs:
Downstream ridging building across Greenland has led to a negative NAO pattern developing:
As a result, rather than our approaching disturbances progressing through the area quickly and moving out to sea, now they will be “trapped” with pooling shortwaves leading to a mean trough setting up in our vicinity. This pattern will likely feature two phases of below normal heights, the first coming late this week through the upcoming weekend, followed by some ridging over the east much of next week before another trough potentially impacts the east beginning late next week. That second one will also have support from the PNA rising to a positive state:
As a result, this will likely lead to a more amplified trough digging down possibly as far south as Florida:
This is all in response to changing pattern, with high latitude blocking forming over Greenland and then also setting up across northern Canada towards the end of the period. Another thing that is interesting to note is that the AO index is now negative, and this is the first time it has gone substantially negative since mid-January:
A negative AO pattern this time of year generally implicates below normal heights in this part of the country:
So, over the coming days, we can expect to see near to slightly below normal temperatures throughout much of the northeastern United States.
Updated: 5:36 AM GMT on May 04, 2011
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