616 fxus62 kgsp 220550 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 1250 am EST sun Nov 22 2009 Synopsis... high pressure over the region will shift north over the weekend. This will allow a low pressure system to move east along the Gulf Coast then north along the Carolina coast Sunday through Monday. The low is expected to reach the New England coast by Tuesday as weak high pressure remains over the southeast states. Another cold front or coastal low could move into the area by the middle of next week. && Near term /through today/... temperatures continue to fall faster than forecast...apart from NE Georgia where the leading edge of the middle level cloudiness is advancing as of 03z. Will adjust low temperatures down a few degrees mainly across the NC foothills and Piedmont...which will not cloud up quick enough to keep temperatures from reaching the old forecast. Otherwise...the rest of the forecast will be left alone. Confluent flow off the New England coast will gradually strengthen surface high pressure over the Middle Atlantic States through Sunday. The morning upper air showed very dry air down along the Gulf Coast. For example...the Tallahassee radiosonde observation had a 40 degree dewpoint depression at 850 mb. With this much dry air to the south...and with a developing classical cold air damming event...it/S not surprising that the models have been delaying the onset of precipitation over the past several runs. At least the GFS has toned down The Pot vorticity maxes and heavy precipitation that it had been driving across The Heart of The Wedge ridge. This seems more realistic as quantitative precipitation forecast should be fairly light with the low level dry air and the weakening southern stream system. The best chances for heavy rain will be along the coastal front...well to our south. Overall...the sref seems to have done the best job of handling the evolution of the Gulf Coast low and precipitation distribution so far. I/ve followed it fairly closely for the timing of precipitation. This means a dry forecast tonight. Precipitation should overspread the region from the southwest on Sunday...though I Don/T see anything falling north of Greenville until the afternoon hours. I raised maximum temperatures a couple categories over the NE zones where thinner cloud cover early in the day will result in a little more warming than originally expected. I followed a met/mav blend and we match up well with the neighbors. && Short term /tonight through Monday night/... as of 130 PM Saturday...not that there is a great deal of confidence with regards to the specifics of the Sunday night forecast...but likely some degree of deep relative humidity and east-southeast flow above lingering wedge will remain all night. Sensible weather is still expected to be damp with rainy and drizzly periods...although precipitation totals will be moderate at worst. Confidence increasing that damming regime will remain through Monday...although forcing for additional precipitation will wane...eventually just being forced by some degree of warm air advection/easterly flow just above surface. Even the milder 12 UTC met numbers...which will probably be too warm anyway...support notable lowering of going maximum temperatures. Given the weakening of the low level flow Monday night and residual weak surface ridging...there seems to be Little Hope of scouring...so will plan on leaving overcast conds with spotty areas of light rain/drizzle... especially along and east of the Escarpment where weak low level easterly flow will continue to lift shallow moisture. && Long term /Tuesday through Saturday/... as of 145 PM Saturday...not much agreement is seen in the op models with respect to amount of deepening of the middle week cut off low and the level of moisture available for the occluded front to work with once it reaches the County warning forecast area Wednesday morning. Even less agreement is seen between the GFS and the European model (ecmwf) on Thursday/Friday with the polar vortex reinforcing the glakes low and the eventual track and speed of the large scale pattern. The GFS 500 mb ens members show the highest spread early Friday morning with about 8 dm of uncertainty associated with the height field...lending low confidence toward the setup for a possible minimal northwest flow event. Not many changes have Ben made to the going forecast with little confidence had both in the op models and HPC guidance. Some decent agreement is seen Tuesday with the slower breakdown of the surface wedge. Model soundings indicate rather thick stratus hanging on throughout the day and very low winds in the bl. So...maximum temperature guidance has been cut back a Cat or so outside the mountains to account for this possible scenario. Probability of precipitation still seem suspect with little in the way of organized flow interacting over The Wedge...so the lingering slight chances were changed little. The GFS is more aggressive in bringing a moist occluded front to the mountains by 12z Wednesday than the European model (ecmwf). The ec does eventually bring the front to the area late in the day...but both models cut off the gom moisture flux with low level ridging from the west. With the main large scale forcing shifted north...will count on very little impact as far a precipitation coverage GOES. For now...slight chances will remain...but this could be increased once better data is gleaned on this system. The next system to think about will be Thu/Fri...as the GFS insists on a moist northwest flow event...yet the ec remains dry and Low Key on the kinematic setup. The GFS as mentioned earlier is bringing in a much stronger reinforcing polar vortex into the glakes low...about twice as strong pvu/wise than the European model (ecmwf). This seems a little too strong for this time of year...and for now will side with the more conservative European model (ecmwf) on this one. Thus...minimal probability of precipitation and an all rain p/type are advertised for that period. Other than the lower than normal probability of precipitation expected Tuesday....Max temperatures should rebound rather nicely Wednesday through Friday. Increased insolation and a warm air advection flow will enable maxes at or perhaps a couple degrees above normal. The NC mountains will be trickery Friday...as it/S probable that cold air advection associated with the passing vortex to the north lowers temperatures throughout the day. Yet for now...will only go a couple degrees below normal with so much uncertainty and poor continuity displayed by the guidance. && Aviation /06z Sunday through Thursday/... at kclt and elsewhere...upstream observation and satellite imagery indicate that scattered to broken clouds in the 10-15 kft layer are just now making their way across north and central Georgia. As such...expect that through late morning all taf sites will nothing more than scattered middle level clouds with broken to overcast cirrus layer and generally light NE flow. Biggest challenge of the present taf period revolves around timing of the lowering of the cloud deck and onset of precipitation. An investigation of soundings across the area indicate that there is substantial dry air from the surface up to around 600 mb...which means it will take a while for even a VFR level ceiling to develop. This brings things down first at kand with MVFR level ceilings and -ra developing around noon. I do bring in MVFR deck at all sites in the afternoon as deep layer moisture increases throughout the day. In fact model solutions show that rapid deterioration of conditions won't occur until after 00z. Models are in pretty good agreement that the greatest coverage of precipitation will occur late in the taf period as a secondary surge of isentropic lift spreads across the region. Will hint at the development of IFR ceilings with a mention of few009 from kand to kclt in the 01-02z time frame. Outlook...ceiling and visibility should come crashing down to IFR and LIFR Sunday night as main precipitation area moves through. Once established...the cold air damming wedge will persist through Monday night with widespread IFR conditions. The model guidance shows improvement Tuesday but most likely at best MVFR should be expected until Wednesday. && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...pattern near term...McAvoy/PM short term...csh long term...sbk aviation...bsh/PM