#1449 Postby Tammie » Fri Dec 26, 2014 5:35 pm
Cavanaugh chimes in on next weeks system. Kinda thought we would be hearing from him after the last AFD.
Fxus64 kfwd 262159 afdfwd Area forecast discussion National Weather Service fort worth tx 359 PM CST FRI DEC 26 2014
Discussion: Early afternoon water vapor satellite imagery showed a large upper trough over the conus rockies with broad ridging over the eastern half of the conus and an amplified ridge centered over the yukon and extending south over the northwestern conus coast. A strong shortwave trough was observed over the far northeastern pacific ocean, south of anchorage Alaska. Locally, visible satellite imagery showed a thick Deck of low clouds over locations along and east of the interstate 35/35w corridor. Just to the west of the interstate 35 corridor, skies were mostly clear. This relatively sharp edge to the cloud cover in place across north and central Texas resulted in a fairly strong west to east temperature gradient across the cwa this afternoon. The temperature at dfw at 230 pm was 60 while the temperature at granbury was 71.
Regional radars and surface observations continue to indicate some pockets of very light rain/sprinkles moving north within the Deck of clouds east of interstate 35 this afternoon. This light rain is expected to persist, but will not amount to much more than a hundredth of an inch of rain through this evening. Between cloud cover and southerly winds of 10 to 15 mph, temperatures are expected to remain mild across much of the cwa until late this evening. A piece of energy aloft embedded with the conus rockies upper trough is expected to eject northeast over the southern plains this evening. This is expected to send the nearly stationary cold front, currently extending from southwest Oklahoma across the southern Texas panhandle, southeast across north central Texas tonight. This front is actually fairly strong, and has a seasonably cold airmass behind it, so it is expected to move across the cwa fairly quickly tonight. temperatures will drop 10 to 15 degrees pretty quickly behind the front, resulting in a much cooler start to the day tomorrow across much of the region. Isolated to scattered rain showers are possible along the front, but rain chances are expected to be slightly higher behind the front tomorrow afternoon. The front is expected to move southeast towards the gulf coast shortly after sunrise, resulting in temperatures that are steady or possibly slowly falling throughout the day.
With the front in place, the last piece of strong energy aloft within the larger conus rockies upper trough is expected to move east over Texas tomorrow. The large scale forcing for ascent induced by this trough will likely bring the best rain chances to the region tomorrow afternoon. The best rain chances seem to exist southeast of a 700 mb baroclinic zone/front tomorrow. This mid-level front is expected to be located near a line from bonham to fort worth to comanche tomorrow afternoon, so went ahead with 30-50 pops southeast of this line, and 20 pops to the northwest. The forcing for ascent does not look overly strong at this time, however a cross sectional analysis of model mass fields indicates that there is a small area of conditional symmetric (csi) to upright instability present just above the sloping 850-700 mb front tomorrow. From a forcing stand point alone, the pops are probably too high as coverage would likely be isolated at best. However with the potential for the release of cape or csi over the area, went with the higher rain chances assuming the weak lift would result in some Decent displacement due to the stability considerations mentioned above. Rainfall amounts will likely remain less than one tenth of an inch on average. Any thunderstorms that are able to develop would result in very localized higher amounts. Precipitation is expected to come to an end from northwest to southeast Saturday night as the upper level energy moves east spreading large scale forcing for ascent over the region.
Clouds will clear out from west to east during the day on Sunday, resulting in mostly clear skies over the region by late Sunday afternoon. Kept Sunday`s highs a bit on the cool side as morning cloud cover, and a late afternoon shift to south winds will prevent a quick warm up from the cool airmass that builds over the region behind tonight`s front. The strong shortwave trough south of anchorage this afternoon is expected to ride up and over the yukon upper ridge this weekend, and then dig almost due south over the western conus coast early next week. With strong ridging over western canada, and a seasonal, deep, somewhat persistent, upper low over the hudson bay, this upper trough diving south early next week is expected to send a strong arctic airmass south into the plains.
All guidance indicates that this arctic airmass will begin surging south on Monday, moving over the southern plains Monday night into Tuesday morning. But, before the arctic airmass arrives, sunny skies and southwesterly winds will likely result in a seasonably warm and pleasant day across north Texas on Monday. Regarding the arctic airmass, it is expected to move across all of north Texas by Tuesday afternoon, however it May move in quicker than that if the 12 and 18Z NAM model runs verify. There is little doubt that a strong arctic airmass will surge south along the high plains on Monday, the main questions for north central Texas are, how fast will it get here? And how cold will it get?
Medium range models notoriously struggle with the speed of arctic air moving south across the high plains. As a result, think that the faster NAM solution is very likely to verify even though it does not represent the consensus of model guidance very well. As a result, went ahead with a cooler solution regarding high temperatures across north and central Texas on Tuesday. Expect strong cold air advection to be ongoing across the entire cwa by the time the sun rises Tuesday morning. How cold will it get? This represents the more complicated question to evaluate as there are multiple ways that arctic air can be modified as it makes its 1500 mile trek south towards north Texas.
The most efficient ways that the brunt of the arctic air is modified or warmed is through downslope winds, and just basic heating by the sun. Along the entire 1500 mile trek south, the arctic airmass will ride along the lee of the rockies, and any westerly winds that impinge upon this airmass could help to slightly warm the air, reducing the strength of the cold air as it continues south. Also, the farther the air sinks south, the greater the sun angle acting upon the arctic airmass, allowing the sun to simply warm up the air the farther south it moves. One of the best ways to evaluate the the potential for the arctic air to warm up as it heads south is to look at model mid-level wind fields.
Westerly mid-level flow over the rockies will induce downslope winds within the arctic airmass. Westerly winds are also more likely to clear out mid to upper level clouds as the basic response to mid-level air moving over the mountains/higher terrain is to sink. Westerly winds do not guarantee clear skies within the arctic air, but it makes it more likely to get some sunshine on the cold air. At this time, the consensus of guidance indicates that mid-level flow will be very weak, or very weak but easterly, as the arctic air moves south along the plains Monday and Tuesday. Easterly mid-level flow would seem to provide optimal conditions for the airmass to build south without warming up as this would result in enhanced cold air damming along the high plains, and would actually support more clouds over the cold airmass.
With seemingly ideal mid-level flow over the arctic airmass Monday and Tuesday, confidence in a strong arctic airmass building over north central Texas is high. Went ahead and bumped temperatures down several degrees from the previous forecast as a result. Raw model 2-meter temperature fields are likely several degrees to warm across the region once the arctic air is in place, but that is very difficult to prove at this point. GFS and ECMWF 2-meter temperatures have been too warm with regards to forecasting most arctic outbreaks for north central Texas from personal experience. that does not mean they are too warm this time, however pattern recognition and past experiences strongly suggest that the region will be colder than what models are advertising at this time.
As a result, have forecast highs and lows from Tuesday through Thursday night that are on the cold side of all available guidance. It is possible that this is not quite cold enough, however cannot reasonably forecast temperatures colder than what any model shows at this time. There is also ensemble support for siding with the colder side of all guidance in this forecast. The naefs advertises a nearly plus 3 standard deviation for mean sea level pressure over the south plains on Wednesday and Thursday on the naefs standardized anomaly page. Also, every member of the 12Z GFS ensemble system indicated a -3 to -10 deg c 2-meter temperature anomaly over north central Texas on new years day. Felt that this was enough support to side with colder temperatures in this forecast.
With high confidence that cold air will be in place across north central Texas Tuesday through Thursday, the attention in the forecast turns to precipitation chances. Unfortunately 12Z model guidance does not offer much in the way of raising confidence in the precipitation forecast for next week. our precipitation chances are tied to the timing of the anchorage shortwave trough moving east over the region next week. 12Z guidance is faster with this trough, however as we`ve seen the past couple of days, models are changing the timing of this upper trough by 24 to 30 hours per model run. This represents the potential for large errors in any deterministic forecast for precipitation until models can start to agree on timing better.
In short, the faster the upper trough moves over north Texas, the greater our chances of wintry precipitation next week. Models are struggling so much with the timing of this upper trough because nearly all guidance indicates that the upper trough will cut off into a closed low by new year`s eve. The farther removed from the polar jet stream this closed low gets, the slower it will tend to move over the region. With the upper trough closed off, there will be no reinforcing of the arctic airmass over the plains for late next week. This will allow the arctic airmass to begin to modify/warm up quickly, likely not posing a threat for wintry precipitation any more by January 2nd.
So, just like yesterday the forecast for wintry precipitation remains uncertain at this time. The forecast confidence remains similar to yesterday. Confidence in low-level air being cold enough for wintry precipitation/ice is highest on Tuesday and Wednesday, and lowest by January 2nd where we will probably be too warm for any wintry precipitation. confidence in precipitation/lift is lowest on Tuesday and Wednesday, and increases quite a bit on January 2nd, when most 12Z guidance moves the upper trough over north Texas. So it is where the confidence in cold air is moderate, and confidence in precipitation is increasing, Thursday, where our chances for wintry precipitation appear to be maximized at this time. There`s little point getting into precipitation type forecasting this far out as precipitation type is usually determined by weather elements not resolved until 48 hours before a wintry event. However, with a shallow arctic airmass in place, and warm air advection above it mid-week, the large scale pattern supports freezing rain more-so than sleet or snow at this time.
Went ahead and placed a mention of rain or freezing rain back in the forecast for much of the region on Wednesday and Thursday as a result of all that has been discussed regarding this arctic airmass so far. it remains entirely possible that the upper trough will simply be too slow and we will just remain cold and dry across north Texas. while that is possible, there are too many wetter solutions that indicate ice is a real threat to leave it out of the forecast entirely at this time. Most guidance indicates that we will warm above freezing by Friday January 2nd, with widespread rain showers likely assuming the upper trough moves over the region at that time. CavanAugh
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Tammie - Sherman TX