Scientific Forecaster Discussion
000 FXUS62 KTBW 170710 AFDTBW Area Forecast Discussion National Weather Service Tampa Bay Ruskin FL 310 AM EDT Tue Sep 17 2019 .SYNOPSIS... 07Z water vapor and H4 RAP analysis showing a highly amplified northern stream upper level pattern over North America. Split flow arriving over British Columbia dives south into a sharp trough along the Pacific Northwest coat before a rapid cyclonic turn northward into a broad and amplified upper ridge extending over the middle section of the CONUS, northward into central Canada. The flow then finally dives back to the southeast, carving out a broad trough over New England and Nova Scotia. It is this slowly amplifying trough that is expected to capture Hurricane Humberto, now spinning of the southeast conus coast, and eject the system northeastward into the north Atlantic. Closer to home, the Florida peninsula resides under a deep layer NW/W flow between Hurricane Humberto and an area of upper ridging in place across the NW Caribbean / SE Gulf of Mexico. The 00Z KTBW sounding profile still showed an appreciable PWAT value over west- central Florida around 1.84" (right about normal for mid-September), however, a much drier column will be arriving from the north/NW within the mean flow through the next 12 hours. The 00Z KTLH sounding from NWS Tallahassee sampled this drier column not far to our northwest with a PWAT value of only 1.18". At the surface, the pressure gradient is becoming rather light as Hurricane Humberto continues to drift east/NE away from the region, and a weak ridge of continental high pressure attempts to build down over the region and eastern Gulf. The radar is very quiet early this morning and with the drier air starting to arrive as we speak, think this quiet nature will continue through the duration of the day. && .SHORT TERM (Today through Wednesday)... Today/Tonight...Benign weather day on tap for the region. A much drier column airmass will be working southward through the Florida peninsula, with most reliable ensemble members showing PWATS dropping below 1.5" for all areas by early this afternoon (some areas considerably below 1.5"). This drier air will keep the forecast rain-free for all areas today and tonight. Not much in the way of clouds will allow for maximum insolation through the afternoon hours, and temperatures are going to respond, reaching the middle 90s for most. Lower 90s can be expected close to the coast where onshore flow develops after 12/1PM. Seasonable temperatures...mainly in the lower to middle 70s are in the forecast for lows tonight. Wednesday...Global guidance showing an interesting flow pattern developing aloft during Wednesday. Stronger flow diving down the eastern seaboard around the backside of Hurricane Humberto is shown to propel a pseudo-backdoor frontal system across the region from NE to SW. This frontal system should be exiting the FL west coast and across the eastern Gulf of Mexico by later Wednesday night. Most ensemble members are showing just enough moisture pooling (PWATS back up toward 1.7-1.8") and focus for ascent along this boundary, especially when it interacts with the west coast sea-breeze, to force a round of scattered late day showers. These shower chances will be greatest near the coast within this enhanced convergence zone. Shower chances then push westward into the Gulf of Mexico with the boundary Wednesday night. A significant intrusion of drier column air is then forecast to arrive in the wake of this boundary for Thursday. More on this drier air in the long term discussion below. .LONG TERM (Thursday through next Monday)... A significant push of dry air arrives from the northeast on Thursday in the wake of a frontal boundary. This airmass will again make rain potential very low across the region. There may be just enough moisture in the very lowest levels to support a few speed convergence showers developing along the FL east coast and moving inland, however, it is unlikely any of these make it far enough to impact Sumter, Polk, Highlands counties...and therefore have PoPs 10% or less for all areas. What may be most significant for us with this airmass is that with good diurnal mixing during the day, we will mix down some rather comfortable air for mid-september. Dewpoints Thursday afternoon are likely to drop into the 60s for many of us. These dewpoints would feel rather pleasant compared to the humid summer air we have experienced for several months. Friday into the weekend we watch a disorganized tropical wave slowly migrate westward toward and through the Florida straits. This system will increase the moisture levels the most across south Florida, and rain chances will be highest well south of I-4. However, the trend has been further north with the moisture return the past few model runs, so have added at least low end rain chances for areas up into Tampa Bay/ Polk county for the weekend (especially in the afternoon). This general pattern persists into Monday. && .AVIATION (17/06Z through 18/06Z)... VFR conditions expected to prevail through the forecast period. Dry air in place will keep rain chances very low. Light variable to north flow through the morning hours becomes northwest 10-15 knots in the afternoon/early evening. && .MARINE... High pressure holds over the eastern Gulf today and tonight with generally light winds and seas. Light winds and seas are expected for most of Wednesday, however, an approaching front from the northeast is likely to bring a round of late day showers and storms. These showers end Wednesday night, but easterly winds quickly increase to cautionary levels Thursday into Friday. A period of advisory level winds and seas is possible Thursday night. Mariners planning to boat on the eastern Gulf later this week should monitor the latest forecast for updates on these anticipated rough conditions. && .FIRE WEATHER... Drier conditions this week in terms of rain chances and dewpoints compared to what we have seen the past several months. A few late day showers, especially near the coast on Wednesday, but otherwise, very low rain chances the next several days. Relative humidity during the afternoon hours will be lower than we have seen lately, but still remain above critical levels, and no significant fire weather concerns are anticipated the next several days. No significant smoke or fog concerns through the middle of the week. && .Preliminary Point Temps/PoPs... TPA 93 75 93 73 / 0 0 40 10 FMY 93 74 92 73 / 0 0 40 10 GIF 95 73 93 72 / 0 0 30 0 SRQ 93 75 93 73 / 0 0 40 20 BKV 94 71 93 71 / 0 0 40 10 SPG 93 76 93 74 / 0 0 40 20 && .TBW WATCHES/WARNINGS/ADVISORIES... FL...None. Gulf waters...None.