Area Forecast Discussion
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898 FXUS62 KTAE 280738 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 338 AM EDT Sun Sep 28 2014 ...FLASH FLOOD WATCH EXPANDED TO INCLUDE ENTIRE FORECAST AREA AND EXTENDED THROUGH MONDAY AFTERNOON... .Near Term [Through Today]... The very weak middle and upper level pressure pattern will remain in place for one more day before a shortwave moving through the Ohio and Tennessee Valleys links up with the cluster of convection in the northern Gulf tonight. Forcing for showers and thunderstorms will likely come from two different features this afternoon. First, convection ongoing over the Gulf this morning will become more organized this afternoon as a weak area of low pressure develops under a weak MCV spawned from yesterdays convection. As the wave of low pressure strengthens, so will southerly flow an isentropic ascent, forcing showers and thunderstorms inland over the Panhandle primarily west of the Apalachicola River. Later in the afternoon, a surface trough will develop over land where temperatures will have climbed into the middle to upper 80s. This feature will bend from the southeast Big Bend, around the central Big Bend and across extreme southern Georgia. Expect a second round of convection to form along this trough in the late afternoon. Eventually, both forcing mechanisms will blend together and a rather solid line of showers, thunderstorms, and heavy rain will spread north through our Alabama and Georgia counties. Satellite derived PWAT values this morning match very well with area soundings and depict that a swath of 2+" has essentially overspread the entire Tri-State region. This evenings Tallahassee sounding showed PWAT values around 130% of normal. With these values expected to only become more anomalous through the day, the threat for heavy rain remains very high. On average, areas receiving rain today should expect to see about an inch, with isolated higher amounts as high as 4 inches. Keep in mind that these amounts will accumulate very quickly and for that reason, the Flash Flood Watch will remain in effect for today and has also been expanded to cover our entire region. .Short Term [Tonight Through Tuesday]... Tough forecast coming up in terms of trying to pin down exactly where the heaviest rainfall will occur in the forecast area and just how much it will be. It is easy to get lost wading through the dozen or so CAMs that we have available to look at now, each with a slightly different (but valuable) take on the situation. Thus, it is best to step back and take a look at the big picture first. The synoptic scale contains two important features. The first is an upper trough currently progressing eastward through Texas, and the second is a very impressive and large plume of tropical moisture with precipitable water values well over 2 inches across most of the Gulf and the Florida peninsula. As the trough continues to progress eastward, a weak area of surface low pressure is expected to develop along the central Gulf coast states and lift northeastward through Monday. The forecast area will also be in the favorable right entrance region of the upper level jet as it lifts north of the area. Checking the forecast soundings from the NAM and GFS for different points in the forecast area, the profiles certainly look tropical with tall, skinny CAPE, high mean RH, and a warm cloud layer. There is also the potential for some training echoes embedded within a larger areas of rain. In fact, we have already seen evidence of training echoes on radar offshore earlier tonight. Thus, many of the large scale ingredients needed for heavy rain will be in place across the entire forecast area as we move into this afternoon through Monday. There is high confidence in the large scale pattern evolution, and in fact the GFS ensemble mean continues to show a bullseye of precipitable water values in the 99th percentile for this time of year squarely in the middle of the forecast area by late tonight, which is signaling a rare event on the large scale. Now we can return to all of those CAM runs with that viewpoint in mind. Looking at the mean of those runs, the highest QPF values through Monday are concentrated right along the panhandle coast and just offshore. This fits with the thinking that a weak area of low pressure will develop along the Gulf coast, which would tend to enhance low level convergence in this area near the coast. Therefore, it seems like the greatest probability of seeing the heaviest rainfall amounts is still along the panhandle coast. However, as the weak surface low lifts northeastward later tonight into Monday, the current belief is that the rest of the forecast area also has a threat of isolated excessive rainfall amounts, especially given the tropical large scale environment that will be in place everywhere in the forecast area. Because of this, we opted to expand the flash flood watch to the remainder of the forecast area and extend it through Monday afternoon. By the time we get into Tuesday, things should be improving as the upper trough is expected to weaken and lift out of the area. .Long Term [Tuesday Night Through Sunday]... The 12z GFS and 00z EURO are in fairly good agreement on the timing of the exiting upper trough and surface low Monday night through Tuesday. However, the GFS is quicker to dry us with deep northwest flow. Will keep a slight PoP in our easternmost zones on Wednesday to account for this. Otherwise, deep moisture begins to return to the region as early Thursday ahead of the next upper trough and associated cold front. The front is forecast to push into our CWA on Saturday. Expect temperatures to run near normal. && .Aviation... [Through 06Z Monday] Generally, a patchy deck of MVFR ceilings will plague the region through the morning hours. This afternoon, we`ll likely remain socked in under a VFR, low to mid- level cloud deck. As showers and thunderstorms spread inland expect IFR to LIFR conditions in very heavy rain. && .Marine... Although winds outside of convection are expected to be in the 10 to 15 knot range today, rather stormy conditions are also likely with numerous showers and thunderstorms. Winds and seas will be locally higher near thunderstorms. Stormy conditions are expected to last through Monday with a decrease in winds and seas expected by mid-week before another front approaches by the end of the week and into next weekend. && .Fire Weather... Hazardous fire weather conditions are not expected over the next few days. && .Hydrology... General rainfall amounts of 1 to 3 inches are likely across the forecast area through Monday, except 2 to 4 inches along the panhandle coast. However, localized amounts around 6 inches in a short period of time appear possible anywhere in the forecast area through Monday given the tropical airmass in place, thus the flash flood watch has been expanded to include the entire forecast area through Monday afternoon. Main stem river flooding is still not expected due to low flows from the recent dry summer and given that any excessive rainfall amounts are expected to be isolated in nature. && .Preliminary Point Temps/PoPs... Tallahassee 85 71 83 72 87 / 60 80 80 30 20 Panama City 80 74 83 70 84 / 80 90 80 30 20 Dothan 81 69 81 66 84 / 80 90 70 20 10 Albany 82 69 81 68 85 / 70 90 80 30 10 Valdosta 87 70 83 70 85 / 80 70 80 40 20 Cross City 87 70 85 70 86 / 70 60 80 40 40 Apalachicola 79 75 82 72 84 / 80 80 80 40 30 && .TAE Watches/Warnings/Advisories... FL...Flash Flood Watch through Monday evening for Calhoun-Central Walton-Coastal Bay-Coastal Dixie-Coastal Franklin-Coastal Gulf-Coastal Jefferson-Coastal Taylor-Coastal Wakulla- Gadsden-Holmes-Inland Bay-Inland Dixie-Inland Franklin- Inland Gulf-Inland Jefferson-Inland Taylor-Inland Wakulla- Inland Walton-Jackson-Lafayette-Leon-Liberty-Madison-South Walton-Washington. GA...Flash Flood Watch through Monday evening for Baker-Ben Hill- Berrien-Brooks-Calhoun-Clay-Colquitt-Cook-Decatur-Dougherty- Early-Grady-Irwin-Lanier-Lee-Lowndes-Miller-Mitchell- Quitman-Randolph-Seminole-Terrell-Thomas-Tift-Turner-Worth. AL...Flash Flood Watch through Monday evening for Coffee-Dale-Geneva- Henry-Houston. GM...None. && $$ NEAR TERM...HARRIGAN SHORT TERM...DVD LONG TERM...BARRY AVIATION...HARRIGAN MARINE...DVD FIRE WEATHER...HARRIGAN HYDROLOGY...DVD is the U.S. government's official web portal to all federal, state and local government web resources and services.