Scientific Forecaster Discussion
000 FXUS64 KJAN 171800 AAA AFDJAN Area Forecast Discussion...UPDATED National Weather Service Jackson MS 1200 PM CST Tue Jan 17 2017 .UPDATE... Updated for 18Z aviation discussion && .AVIATION... 18Z TAF discussion: MVFR to VFR conditions will continue through the afternoon. Scattered showers this afternoon will be most concentrated along a slow moving cold front extending from KIER-K5A6- KTUP line at 17/18Z. After 18/00Z, IFR conditions will increase as low clouds and fog develop. LIFR conditions will develop once again at HBG as the fog becomes locaally dense./26/ && .DISCUSSION... Today and tonight...An upper level low and it's attendant surface low, will continue tracking northeast into the Great Lakes Region today. This will cause a cold front to shift east and into the forecast area. As this front loses its upper level support, it'll eventually become quasi-stationary on a northeast to southwest trajectory across the forecast area tonight, in the general vicinity of the Natchez Trace Parkway. Aloft and over us, persistent ridging wedged into the region from the east southeast will maintain southwest flow, thus increasing moisture aloft over the region. At the surface and ahead of the front, south to southwest winds will also result in a continual stream of moisture over much of the forecast area. All of the above, in addition to a couple of weak impulses embedded in the before mentioned southwest flow aloft traversing the region, will result in good rain chances across much of the forecast area today and into this evening. Overnight, while there will be some some showers across the CWA, there should be a an overall lull in activity. However, another disturbance aloft is progged to begin impeding upon the region from the southwest Wednesday morning. Some storms will also be possible across the CWA today, primarily along the front itself. However, these storms are expected to remain well below severe limits. That said, some heavy downpours will be possible with the activity ahead of and along the front, with some training of convection possible. Behind the front, west northwest winds will advect in some slightly cooler drier air. Otherwise, highs across much of the forecast area will top out in the low and middle 70s. The only exception being in the Delta were low to upper 60s are expected behind the front. Lows tonight will range from around 50 in the Delta, to the lower 60s across South Mississippi. /19/ Wednesday through Monday...It will continue to feel more like late March or early April than mid January in the ArkLaMiss this period. Anomalously warm and stormy weather will be the general rule as a strong southern stream upper level jet interacts with unseasonably moist and unstable airmass surging northward into our region. The most impactful weather should center on Thursday, when there will be a limited threat for flooding and severe weather, and from Saturday into Sunday when there will be the potential for a more significant severe threat. Wednesday should see somewhat of a lull in the action as the frontal boundary stalls and weakens over our area beneath a continued strong southeast CONUS ridge while greater forcing and moisture convergence refocus to the west with the approach of more meaningful height falls. Expect the potential for heavier convective rainfall to develop Wednesday night over the Delta region as low level flow increases to 30kts helping to transport a 1.5+ inch precipitable water airmass northward while most unstable CAPE increases to ~500 j/kg. This heavy convective rainfall potential will shift east across the remainder of the forecast area Thursday with increasing deep layer shear and instability supporting the possibility for a few strong to severe storms especially along/se of the Natchez Trace corridor. It looks like the widespread nature of clouds/precip will limit instability and overall severe threat. The moisture convergence axis should achieve longest duration over roughly the same area resulting in heavy rainfall as well. The long spell of drier than normal conditions and unseasonable warmth have reduced soil moisture so that flooding is not a big concern for now...but localized rainfall rates could certainly become heavy enough for at least minor flooding given the anomalous moisture. The trough axis will sweep activity quickly east of the area late Thursday and expect another break in the action Friday into Friday night. Global models continue to indicate that a more significant threat for severe weather should evolve this weekend. Expect rapid onset of moisture transport early Saturday into southern portions of the ArkLaMiss, and with steepening mid level lapse rates > 8 deg C/km and H850 theta-e values ~330 K, mixed layer CAPE is forecast to increase to near 2000 j/kg, an extreme value for January. Along with strong deep layer shear, there is little doubt that the environment will be supportive of significant severe weather. But due to the very fast upper level flow and how surface features will respond, there is a lot of uncertainty regarding lift and specific areas of deep convection, especially with the tendency for coastal convection in this sort of pattern with big height falls well to our south. After coordination with SPC, will indicate a limited severe threat for mainly the southern half of the area and hold off on elevating the threat until model details can become more consistent with convective evolution. Will also broad brush the time window to include both Saturday/Sunday. Look for quieter and cooler weather beyond Sunday as the first significant intrusion of CP air tries to move into the region. /EC/ && .PRELIMINARY POINT TEMPS/POPS... Jackson 76 60 71 63 / 25 32 38 48 Meridian 76 60 72 61 / 15 25 25 36 Vicksburg 71 57 68 62 / 47 38 57 62 Hattiesburg 78 61 74 63 / 16 12 25 27 Natchez 75 61 72 64 / 36 28 53 58 Greenville 61 50 61 56 / 65 52 71 70 Greenwood 67 52 65 61 / 81 49 56 63 && .JAN WATCHES/WARNINGS/ADVISORIES... MS...None. LA...None. AR...None.