December 2016- End Of Year Forecast

General Weather Discussions and Analysis

Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby unome » Tue Nov 29, 2016 6:13 am

the uncertainty in the models makes it a good time to pay close attn to local forecasters, looking forward to later discussions:

http://www.weather.gov/hgx/
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The cooler/drier weather in the wake of this front for Weds/Thurs is expected to come to an end Friday. The return of onshore winds and low-level moisture will be setting the stage for a wet/active weekend. A deep closed upper low/trof (developing in the vicinity of the California Baja) is progged to move east across nrn Mexico Sat/Sun with a coastal surface trof setting up over the middle to lower TX coast out ahead of it. These systems combined with possible PWs near 2" are all pointing to widespread unsettled weather for most of the weekend with heavy rain as the main issue. Models appear to be in slightly better agreement with these runs, but we are still seeing the biggest differences with timing. So for now, stay tuned. Will be adding some initial wording for this weekend in the HWO later this morning.


http://www.wpc.ncep.noaa.gov/index.php#page=frt

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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby srainhoutx » Tue Nov 29, 2016 8:48 am

Tuesday morning briefing from Jeff:

Significant rainfall event increasingly likely this weekend over SE TX.

Surface boundary stalled along the US 59 corridor yesterday evening and has returned slowly northward overnight in response to an incoming short wave over the Rockies. South of this boundary dewpoints are in the 70’s while north of the boundary dewpoints are in the 40’s and 50’s and this has resulted in widespread dense fog near the boundary. Recently radar has shown a few showers starting to develop over the region and this will continue through the morning and into the afternoon hours with modest heating along the boundary and a secondary stronger incoming cold front. Any severe weather should be east of the area…but a thunderstorm or two will be possible today.

Cool and dry across the region Wednesday-Friday with lows in the 40’s and highs in the 60’s.

Weekend:
A powerful upper level storm system will drop into the SW US/N MX late this week and slow down this weekend. Increasing SW flow aloft will help to foster the formation of a surface coast trough from the upper TX coast to the lower TX coast where surface convergence will be maximized. NE surface winds will lock in cold surface temperatures in the 40’s and 50’s while a few thousand feet off the ground warm moist southerly flow will overrun the surface cold dome…the result will be widespread rainfall…some heavy.

While models continue to disagree on the timing of this feature with the GFS/CMC/UKMET the slowest and the ECMWF the faster…the all agree that some significant QPF is looking likely with this event. The GFS has been producing 3.0+ inches at BUSH IAH for the last several runs in a row. Given forecasted PWS approaching an extremely high 2.0 inches feel the threat for heavy rainfall is growing for this weekend. Main question is does the active convection focus near the surface trough off the coast or in an elevated fashion inland of the coast. Both are possible at this point…but the results would be vastly different QPF.

For now will go with widespread rainfall of 2-4 inches starting late Friday and lasting into Monday with isolated amounts of 6 inches possible. This is falling in line with current WPC forecasted amounts. Should the coastal surface trough back northward or form closer to the coast this would increase the threat for prolonged cell training of heavy rainfall.

The entire period will be cold with temperatures hovering in the 40’s to lower 50’s much of the weekend as NE surface winds drain cold air into the region under rain and clouds.

Gatlinburg/Pigeon Forge Wildfire Complex:

An incredibly devastating night for the resort areas of eastern TN as at least 14 wildfires driven by 50-70mph winds burned through Gatlinburg, Sevierville, Kodak, and Pigeon Forge/Dollywood. The magnitude of the fire that swept across this area yesterday afternoon and evening is rare for the eastern US. This exact event is what has been feared for so many weeks as crews have battled to gain control of the many fires burning in the Great Smokey Mountains in fear that weather conditions would change rapidly (as is normal for this time of year) …yesterday was that day.

The same low level jet that was roaring overhead of SE TX yesterday moved over E TN late yesterday afternoon with frequent surface wind gust of 50-70mph. Fires that had been burning in the Great Smokey Mountains to the south of Gatlinburg for weeks exploded even with RH values of 50% or higher being driven by intense surface winds. At 400pm the fire complex was nearing Gatlinburg from the SE/S/SW.

Between 400pm-1100pm the wildfire erupted into a firestorm with forward burn rates of nearly 50mph at times racing the fire northward through large sections of Gatlinburg and into Pigeon Forge. Crews working fire lines to protect the towns during the day were rapidly overrun and tremendous spotting of embers in the strong winds resulted in the initial fire expanding into at least 14 different fires in the matter of a few hours. Air support was grounded due to the strong surface winds and ground crews Fire rapidly overran the primary evacuation route out of Gatlinburg…see images below. Visitors in many of the areas hotels were trapped in the hotels as fire rapidly surrounded many of the structures.

Radar data showed nearly 45dbz in the smoke plume indicating large amounts of ash and objects were being lofted into the air suggesting the fire had developed surrounding inflow winds and strong center updrafts…something only seen in the most intense of wildfires. To give a picture of how quickly the fire evolved attached is a weather station in Gatlinburg which was overrun by the fire. The surface temperature spiked to 118 degrees and the RH fell to 21%...clearly the fire was creating its own weather as other sites nearby were reporting 50-60% RH values. Also note the peak wind gust of 69mph about 2 hours before the site was overrun…showing intense surface winds near the fire line. Note the inside structure temperature was 64.9 degrees…clearly the fire was advancing on the structure at this moment…and this was the last report from this site.

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Officially 100 structures were lost in the areas surrounding Gatlinburg and Pigeon Forge including a 16 story hotel, but the scope of the devastation will not likely be known until today when the sun rises…some estimates suggest nearly 1/3rd of the town of Gatlinburg burned overnight. As one meteorologist said this morning “not since the Great Chicago Fire has a combination of weather events, combined to cause such an event east of the Mississippi River”

Evacuation Traffic Being Overrun by fire on HWY 441

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Wind driven spot fire on primary evacuation route out of Gatlinburg…HWY 441
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Downtown Gatlinburg:
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Fire advances on Gatlinburg out of the mountains
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Gatlinburg:
11292016 Jeff 7untitled.png
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby MontgomeryCoWx » Tue Nov 29, 2016 11:53 am

12z GFS brings quite the Arctic blast early next Thursday morning.

Would not get out of the 30s in MoCo on Thursday followed by a couple of freezes.
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby srainhoutx » Tue Nov 29, 2016 12:19 pm

MontgomeryCoWx wrote:12z GFS brings quite the Arctic blast early next Thursday morning.

Would not get out of the 30s in MoCo on Thursday followed by a couple of freezes.


Looks like the first true "Bue Norther" we have seen in a couple of years. We will need to watch the Winter Storm that develops to our S and W and moves into the Plains laying down a bunch of snow. If we see significant snow cover develop across the Central and Southern Plains, that tends to suggest less in the way of airmass modification for any future cold air intrusions. Stepping down... ;)
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby srainhoutx » Tue Nov 29, 2016 1:23 pm

The 12Z GEFS Ensemble Mean agree with the other guidance suggesting the first real shot of Arctic air surges South late next week.

Still looking raw, stormy and wet this coming weekend.
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby srainhoutx » Tue Nov 29, 2016 1:55 pm

the 12Z ECMWF suggests a bit slower ejection of the cold core upper low this weekend and has trended toward the slower and deeper solutions with the trough to our West. That would mean the heavy rainfall potential could increase a bit with better agreement with the other guidance and the ECMWF shows a developing Coastal trough/low as well.

11292016 12Z EC 120 f120.gif


11292016 12Z EC 144 f144.gif


In the extended range, the "Blue Norther" is still showing up very well for arrival a week from tomorrow.

11292016 12Z EC 192 850t_anom_na.png
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby srainhoutx » Tue Nov 29, 2016 4:54 pm

The weekend forecast is becoming increasingly worrisome, but at this range still lacks confidence. The Global Models and their ensemble members have come into better agreement regarding the very deep trough that looks to settle well into Mexico near or just East of the Baja Peninsula. A very potent cold core low at the 500mb level is expected to deepen over Mexico and tap a very noisy sub tropical Jetstream with abundant tropical moisture at the mid and upper levels reaching back to near Hawaii. That moisture appears to have several embedded disturbances that will round the base of the deep trough with a couple of shortwaves dropping S along the Western flank of the trough along the Washington/Oregon/California areas adding some energy to the potent cold core low. Overrunning precipitation is expected to develop atop a shallow cold airmass at the surface Friday night into Saturday with a strong SW flow aloft. The reliable models have agreed today that a Coastal low/trough could organize near Brownsville to Corpus Christi with the potential of a Coastal low developing Saturday night into Sunday and possibly next Monday. Elevated thunderstorms are possible Saturday night into early Monday as the Coastal low moves along the Upper Texas and SW Louisiana Coast. The upper low looks to begin shearing out Sunday evening into Monday as it moves across Texas. The afternoon Update 7 Day Quantitative Precipitation Forecast suggest bouts of heavy rainfall may be possible, particularly along and offshore of the Texas Coast. This is a very complicated and challenging forecast, so expect changes as we near the end of the work week.
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby DoctorMu » Tue Nov 29, 2016 5:56 pm

Freezing down to the HOU area next Friday am (Dec 9)...off in fantasyland. Waiting for 18z run to complete.

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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby MontgomeryCoWx » Tue Nov 29, 2016 6:00 pm

Long range keeps reloading cold air in our source region and putting it on the shot block down to our smiling faces
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Re: December 2016- Cold/Rainy Weekend. Tracking Arctic Air

Postby Andrew » Tue Nov 29, 2016 6:01 pm

Looks like an active period for SE Texas will occur over the coming weeks. The cutoff low/coastal low combination this coming weekend looks tricky as it will be a battle between the upper dynamics to the west and the coastal low to the east. On multiple occasions we have seen the coastal low "steal" much of the moisture that would make it more inland. As it stands right now I do expect locations along the coast will see highest totals and a relatively sharp gradient will exist as we move inland. With that said, the ECMWF is more encouraging with moisture streaming further inland (less influence from the coastal low) and isolated totals reaching 5-7 inches inland. Overall though much needed rain looks to occur for most across the region.

Next, the artic outbreak is looking more and more likely for the December 9th timeframe as both global models continue to keep the AO in a negative state. The ECMWF is most aggressive with the colder air showing dewpoints reaching close to the single digits over parts of SE Texas. The GFS keeps dewpoints in the mid to upper teens though. Still a longways out and modification and proper adjustments will need to be made.
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