Preliminary Forecasts
Issued by NWS
Issued by NWS
719 FXUS02 KWNH 120713 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 312 AM EDT Sun May 12 2024 Valid 12Z Wed May 15 2024 - 12Z Sun May 19 2024 ...Another southern tier heavy rainfall threat mid-late week... ...Overview... A system reaching the Mid-Atlantic by early Wednesday and a cold front just to its north/west will support a wet midweek over portions of the East before the surface low continues eastward into the Atlantic. Farther west, shortwave energy emerging from the Rockies should support a wavy frontal system pushing eastward from the Plains, spreading another broad area of rainfall over the central/eastern U.S. with highest totals still expected to be across the southern tier. Guidance is still having difficulty in resolving some important details though. Potential stalling of the system`s trailing front near the Gulf Coast could support at least a localized heavy rainfall threat into the weekend. Models/ensembles still differ for specifics of flow rounding eastern Pacific ridging from Wednesday onward with effects translating eastward with time, but at least the prior extremely broad solution envelope has narrowed somewhat. ...Guidance/Predictability Assessment... Comparisons of 12Z/18Z dynamical guidance and ECMWF-initialized machine learning models, with different solutions clustering better for different features, ultimately led to an even weight of the GFS/ECMWF/UKMET/CMC for about the first half of the period and then a transition to 30-40 percent total 18Z GEFS/12Z ECens weight while maintaining input from the GFS/ECMWF/CMC. For the system tracking off the Mid-Atlantic, GFS runs in particular have had more stray solutions recently compared to other guidance and the ensemble means (with consensus saying the upper dynamics should stay more separated from flow to the north). 12Z/18Z runs were somewhat north and fast but the new 00Z run fits with the means better, while now the 00Z UKMET strays to the north side of the spread. Remaining details of the forecast remain ambiguous, with a combination of differences for how shortwave energy emerging from the Plains/Rockies evolves, details of a separate southern stream shortwave tracking out of the Southwest (plus potential for additional lingering energy that could eject later), and flow coming into North America around the top of eastern Pacific ridging. Amidst all these detail issues there is at least a general theme of the flow evolving toward a more warm-season pattern with the westerlies becoming better defined along the northern tier U.S. Regarding flow around the Pacific ridge, guidance has been trending toward an intermediate solution over the Northwest mid- late week, with upper ridging holding on a little longer than the average of prior spread (but not as strongly as some GFS/CMC runs) and then staying near the middle of the envelope later in the period. Some ECMWF-initialized machine learning models suggest the system tracking along the Canadian border late week into the weekend could be a little stronger than the dynamical model average. Farther east, surface details will depend on lower- predictability details of shortwave energy. Latest guidance has shifted northward with the main area of low pressure after reaching the Mississippi Valley by early Friday but then consensus suggests the triple point wave may become the primary low upon reaching the western Atlantic. ...Weather/Hazards Highlights... The primary focus for heavy rainfall potential still looks to be in the southern half of the Plains by the Wednesday-Wednesday night time frame covered by the Day 4 Excessive Rainfall Outlook. Reflecting some developing spread for approaching dynamics aloft (from the Rockies, as well as a separate Southwest shortwave that may have some interaction with northern stream flow), model/ensemble guidance continues to differ for convective specifics over this region. The GFS/ECMWF and their ensemble means maintain a theme of instability and increasing moisture over this region, along with multiple surface features that may help to focus convection. There has been some lingering support for one area of emphasis over Texas where ground conditions are on the wetter side so for now the ERO maintains the Slight Risk area close to continuity. However there is a developing signal from some dynamical and machine learning output toward better convective emphasis a little farther north--over an area where short term forecast rainfall could lead to wetter soils heading into Day 4 than currently observed. Prefer to wait another run to gain better confidence in any potential adjustments. Meanwhile continuity looks good for the lingering Marginal Risk over northern Florida as the front reaching the area stalls, accompanied by instability and above normal moisture. For the Day 5 ERO (Thursday-Thursday night), aforementioned spread for details aloft continue to influence convective details from the Plains into the Gulf Coast states. The outlook starts with a Slight Risk area extending from southeastern Texas into southwest Mississippi, which represents the average of ensemble probabilities for greatest rainfall (or specifically the 12Z ECens, with the CMCens a little southeast and the GEFS a little north) along with latest ECMWF/CMC runs plus 12Z machine learning models. Even though GFS details aloft lead to much less QPF, it is more similar to other models/means in depicting a pronounced surge of moisture into this region with adequate instability as well. A front farther north may help to focus less extreme but still locally heavy rainfall, which is accounted for by a Marginal Risk area that extends into parts of the south- central Plains and Midwest. Expect the moisture shield to continue across the eastern U.S. late in the week, with heaviest rainfall most likely extending across the Southeast. Depending on specifics of upper flow, the front trailing from the surface system associated with this episode could stall near the Gulf Coast and provide a focus for additional rainfall during the weekend. An eastern Pacific upper ridge extending into the West should support above normal temperatures over most of the West Coast states and parts of the Great Basin during Wednesday-Thursday, with some areas seeing plus 10-15F anomalies. Thereafter, expect the warm readings to persist over California/Nevada with eventual expansion into the southern Rockies while the Northwest trends closer to normal (possibly below normal over Montana). Most of Florida may see several days with highs up to 5-10F above normal, in a warm pattern to the south of a couple fronts expected to stall upon reaching the far northern part of the state. Combined with high dew points, these temperatures may bring heat indices as high as 105-110 across southeast/southern Florida, signaling the first hazardous heat days of the summer. Otherwise, areas to the east of the Rockies will tend to be within a few degrees of normal for highs, aside from below normal over the southern Plains on Thursday and possibly an axis of modestly above normal readings dropping southeast from the northern Plains Friday onward. Most of the eastern U.S. should see somewhat above normal lows through the period. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw $$