Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Thu May 16 2024 Valid 12Z Sun May 19 2024 - 12Z Thu May 23 2024 ...Overview... Reasonably agreeable model guidance indicates that an upper trough and surface low should move out of the eastern U.S. but linger in the western Atlantic through the first half of next week. Meanwhile, mean troughing will develop in the western U.S., which along with frontal boundaries should help develop some locally heavy convection across north-central parts of the Plains and Mississippi Valley toward the Great Lakes region early to midweek. Leading shortwave energy looks to push across the northern two- thirds of the U.S. along with a cold front Tuesday-Thursday, while an upper low drops south into the Northwest. Cool conditions with increasing precipitation chances are forecast for the Northwest, including higher elevation snow. ...Guidance/Predictability Assessment... Recent model guidance is generally agreeable with the overall pattern, with some typical detail uncertainties. There is still some spread with the specifics of the eastern U.S. to western Atlantic low, but the 00Z UKMET seemed to be the main outlier with it in showing a farther east/progressive solution more reminiscent of models from a couple days ago, so a GFS/ECMWF/CMC blend worked well. Farther west, models indicate a few features of note that combine into the western U.S. trough. A small southern stream eastern Pacific closed low Sunday will track eastward and get absorbed likely Monday-Tuesday, while northern stream energy pivots through the northwestern and north-central U.S. within the trough. 00Z/06Z model guidance seemed agreeable with that but the new 12Z CMC now lingers that energy in the West longer while other models progress it into the Midwest. The new 12Z CMC also seems like an outlier upstream as it does not show the upper low dropping south through the western side of the trough by midweek. The WPC forecast used a blend of the 00/06Z GFS and 00Z ECMWF and CMC runs early in the period, with gradually increasing incorporation of ensemble means to about half by the end of the period. This maintained good continuity with the previous forecast. ...Weather/Hazards Highlights... During the Sunday-Monday night time frame covered by the Days 4-5 Excessive Rainfall Outlooks, the primary region of interest will extend from the central Plains into the Midwest. A Plains surface low should lift an initially stalling front northward as a warm front while a wavy boundary should hang up for a time over the Plains/Upper Midwest early next week while awaiting the approach of western U.S. dynamics. There is still some scatter for details of the placement for possibly multiple rounds of thunderstorms in this area, but the ingredients of ample moisture/instability and lift warrant continuing Marginal Risks in the EROs for both Days 4 and 5, with threats on Day 5 depending somewhat on how Day 4 transpires. From early Tuesday onward, expect northeastward progression of developing low pressure to push the trailing cold front and accompanying rain and thunderstorms of varying intensity farther east and south. Rainfall amounts could be heavy across the Midwest into the Great Lakes region, but other than that it will take additional time to resolve most favored areas of heaviest rainfall in the eastern and southern U.S. for the Tuesday-Thursday time frame. Aside from the north-central U.S., there is some concern Sunday for locally heavy rain in the Southeast to southern Mid-Atlantic with the departing low pressure system, but without enough of a signal to warrant a risk area. Then in the West, initial upper troughing will support some precipitation over northern areas with scattered activity possibly reaching as far south as the Great Basin. Some snow will be possible in the high elevations of the Rockies, along with high winds possible on Sunday. Approach/arrival of another upper low as currently advertised by midweek or so would produce somewhat more organized precipitation at that time. Changes in snow levels will depend on the depth of the upper low. Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near or over 100 degrees could stretch farther north across the southern High Plains at times. Some daily records for highs/warm lows will be possible. Southern Florida may also experience hazardous heat into Sunday with highs locally up to 5-10F above normal and max heat index values reaching 105-110F, followed by continued above normal but slightly less extreme readings. The forecast pattern will favor below average highs over the Northwest for most of next week, and including the northern Plains by early next week. The warm sector from the High Plains into the Great Lakes/New England early in the week should see highs up to 10-15F or so above normal. Cold frontal progression should bring temperatures closer to normal over most of the eastern half of the country by mid-late week. Clouds and possibly lingering pockets of rainfall should keep highs on the cool side over the Mid-Atlantic during Sunday-Monday. Tate/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