AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 354 PM EST Tue Feb 21 2012 SYNOPSIS... The long wave pattern is continuing to progress. The RIDGE that was over the SE US yesterday has moved east over the MID-Atlantic. Currently over our region is a SHORTWAVE TROUGH that brought some light RAIN this morning and some CLOUDY skies. Besides a FEW light showers this morning, today has been a CALM day in terms of weather. IR and visible imagery show mid-upper level clouds. RADAR is QUIET with a few leftover showers on the outskirts of the forecast area. The trough should continue to progress eastward, but more slowly than yesterday`s ridge AS it will begin lifting and our region will begin to see more zonal FLOW until a second LOW approaches Thursday NIGHT. This low could render some weather on Friday. && .NEAR TERM [Today and Tonight]... Heavy CLOUD cover and light rain from this morning have made us lower today`s forecast HIGH today to the mid to upper 60s. Skies are expected to clear for a little bit with a short break in the cloud cover. With light southerly winds overnight and high MOISTURE from RAINFALL and ADVECTION by said winds, FOG is expected across most of the area starting early tomorrow morning and continuing into the afternoon. Model forecast soundings and statistical guidance indicates that the fog could be dense in spots. && .SHORT TERM [Wednesday through Friday Afternoon]... Plenty of weather concerns in the short term period, as nearly 48 hours of low-level warm advection and associated various rain chances will be followed by the arrival of a strong cold FRONT on Friday. We edited the GRIDS today out through 00z Saturday to ensure a more smooth transition of weather elements into the previous long-term forecast from last night. First, model preferences. Differences have re-emerged from what was a fairly good consensus amongst the 21.00z model suite. This seems to be largely tied to two features: (1) the upper-level CUTOFF low that is centered near 28N/117W or just west of Baja California, and (2) an elongated PV ANOMALY / axis of SHEAR VORTICITY north of a strong JET streak in the Gulf of Alaska. The general theme amongst the models is that the shear vorticity will consolidate and dig southeast into the Plains by Thursday as the jet streak develops eastward along the NE rim of the Eastern Pacific ridge. This is expected to lead to low-level CYCLOGENESIS in the Great Lakes or Ohio River Valley on Thursday, and will aid in driving a deeper cold AIR mass southeast behind a surging cold front. The new 12z GFS is much faster with this cold front, pushing it through our area by 18z Friday, whereas the global model consensus (and prior 00z GFS) was much slower. In collaboration with surrounding WFOs, TAFB, and HPC, we have decided to go with the slower cold front timing, with most weather elements a blend of 21.00z GFS, 21.12z NAM, and 21.00z ECMWF. There are also major differences in how the models handle the Baja low, with potential implications for the long term period (weekend and beyond). That will be sorted out on the overnight shift by the long term forecaster. For tomorrow, there is good agreement on a quick round of rain with a fairly sharp N-S POP/QPF GRADIENT, with highest values south. This seems to be forced largely in the low levels as isentropic ascent kicks in as a response to a subtle shortwave trough ejects east from the Rockies to the east coast in fast, nearly-zonal flow. Arguing for greater rain coverage is the added benefit of being in the right entrance region of a 120-130kt upper level jet streak. High-res model guidance is in excellent agreement that showers will affect mostly our marine and Florida zones, entering the western half of our area mainly 14-18z, and then moving quickly east and diminishing by early Wednesday evening. The various 4km WRF models all indicate some small amounts of CAPE with some weak-moderate UPDRAFT velocities out over the Gulf, so THUNDER was added over the water. The earlier prospects of SCATTERED thunderstorms on Thursday seem to have dwindled on the latest model guidance. As we are LIKELY to warm into the upper 70s and potentially near 80 degrees, we should see at least a weak sea-breeze CIRCULATION develop by the afternoon. 12z model guidance does seem to support this notion with some light QPF (0.01-0.05") amounts hugging the coastline during the day. Forecast soundings don`t indicate much INSTABILITY, with LCL-300mb lapse rates almost at moist ADIABATIC levels. The SREF model MEAN SBCAPE is around 300 j/kg with a few members up around 600-800 j/kg. There will also be quite a bit of WIND shear, with 0-6km bulk shear around 60 knots. Therefore, the threat of strong to severe thunderstorms is non-zero. One potential flaw is strong WAA in the 900-800mb layer just atop the boundary layer, particularly in the afternoon. This could limit updraft strength and keep CONVECTION mostly as shallow showers. Given the amount of shear, there are a wide RANGE of convective possibilities on Thursday, so stay tuned! It looks like we will see a lull for most of Thursday Night as overnight storms would tend to fire to the northwest of us along the cold front in the Mid-South. With the strong WAA regime and likely increase in low cloud cover, it should be a warm night with lows in the mid 60s. For Friday, severe weather is a possibility and we are currently outlooked in the Day 4-8 OUTLOOK" onClick="return popup(this, 'notes')">OUTLOOK with severe probabilities at or above 30%. More details on this in the coming days, but the main points with this forecast update are: (1) the timing of the greatest severe weather threat appears to have shifted into the daytime on Friday, (2) a DIURNAL timing of the storms would probably favor a bit more instability, (3) regardless of eventual timing the ATMOSPHERE will be highly sheared. While there are differences in the models regarding timing and some MESOSCALE details, they all agree that there will be an arc of thunderstorms along the front. .LONG TERM [Friday Night through next Tuesday]... On a more positive note, the cold front and associated thunderstorms will be exiting quickly to our east Friday evening setting up a dry...although cooler weekend period. A more zonal upper level pattern over-TOP surface high PRESSURE building in from the west supports near zero rain chances Saturday and Sunday. The airmass arriving is quite chilly through. The late February sun is getting stronger, so still anticipate highs into the 60s, however a chilly night looks to be in store for Saturday night. As of now have gone with WIDESPREAD mid/upper 30s away from the immediate coast, with normally colder spots down near freezing by SUNRISE. This will also need to be monitored closely, as the eventual position of the surface high Saturday night will determine if a more widespread light FREEZE will be possible. Seasonal conditions with rebounding TEMPS then expected for the early portion of next week. && .AVIATION [through 18z Wednesday]... 12Z model guidance is showing lower CIGs and VISBYs for the BR Wed morning and conditions deteriorating earlier for ABY and DHN than forecast at the 12Z TAF issuance. BR setup time remains similar to the last issuance, beginning around 04Z, deteriorating near 09Z, and improving at 16Z, earlier at ECP and VLD. Model agreement is quite strong on low VSBY, but weak on CIGs, especially after 14Z. Expect VFR conditions until around 04Z Wed when conditions FALL to MVFR. At 09 conditions will fall further to IFR, then return to MVFR around 15Z. && .MARINE... Generally quiet marine forecast until at least Wednesday. A NOCTURNAL SURGE of southerly winds could REACH" onClick="return popup(this, 'notes')">REACH SCEC headline levels Wednesday Night, subsiding briefly on Thursday, before ramping up ahead of a cold front Thursday Night into Friday. We went entirely with a SEAS forecast based on the SWAN model, as the WW3 (based on the 12z GFS) seems to be: (1) too fast with frontal timing, and (2) too weak with surface winds. The winds were a blend of the ECMWF and NAM models which bring the front through the coastal waters on Friday afternoon. Ahead of the front this gives SSW winds around 25 KT with seas building to 7-9 feet. The combination of the rapid ramp-up in wind-waves, plus longer period SWELL originating from the 15-20kt SSW flow Thursday to Thursday Night is expected to build surf heights to ADVISORY levels on Friday (6ft or so), with dangerous rips likely. && .FIRE WEATHER... High relative HUMIDITY values keep us well above criteria level across the region through the end of the work week. Relative humidity is forecast to decrease to around 30 percent on Saturday across the area for several hours, so Florida may reach red flag criteria, but it`s too early to see ERC values, or to be certain of what the winds will do.