Preliminary Forecasts
Issued by NWS
Issued by NWS
328 FXUS02 KWNH 180704 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Tue Jun 18 2024 Valid 12Z Fri Jun 21 2024 - 12Z Tue Jun 25 2024 ...Record-breaking heat from the Midwest to the East Coast expected to moderate by early next week... ...Heavy rainfall threat from the northern Plains into Upper Great Lakes into Friday-Saturday... ...Overview... Latest models and ensembles continue to show the large scale pattern transitioning toward a more typical early summer regime late weekend and early next week, as initially strong ridging over the eastern half of the country gives way to a leading trough crossing the northern tier U.S. and southern Canada (reaching the East Coast by around next Tuesday) while a system reaching the Northeast Pacific and West Coast this weekend continues eastward thereafter. Although weakening, the eastern upper ridge will still support a broad area of hazardous heat with potential for daily records extending from the Ohio Valley/Lower Great Lakes into the Mid-Atlantic Friday into the weekend. The northern Plains into Upper Great Lakes should see a continued threat for heavy rainfall through Friday-Saturday as developing northern Plains into Great Lakes/Ontario low pressure and associated fronts interact with anomalous moisture already in place. Some of this rainfall will extend into the Northeast. A trailing Northeast Pacific system will likely bring a well-defined front into the West by Sunday, continuing east thereafter. To the south of the upper ridge ultimately settling over the far southern tier, Potential Tropical Cyclone One (see NHC for updates) should dissipate over Mexico by Friday while NHC is monitoring potential for another feature over the western Gulf mid-late period. ...Guidance/Predictability Assessment... The latest forecast based on 12Z/18Z guidance started with an operational model composite early in the period and then gradually increased ensemble input (18Z GEFS and 12Z ECens/CMCens) to a total of 60 percent by the end of the period next Tuesday. ECMWF input was split between the 12Z and 00Z/17 run mid-late period due to some questionable details in the 12Z run. At least in principle, an average of dynamical and ECMWF machine learning (ML) guidance agreed fairly well with the upper ridge settling over the far southern tier while a fairly vigorous system develops/tracks along the northern tier/southern Canada through the period, leading to a moderate upper trough/leading cold front near the East Coast by next Tuesday. The mean of ML models was close to the dynamical guidance average for the surface system`s depth, while displaying some detail differences after most guidance agreed to an embedded upper low aloft as of Sunday. With the trailing Pacific system, sporadic GFS runs (like the 12Z and new 00Z versions) have been shearing some upper energy while leaving the main upper low behind near the British Columbia coast. The 18Z GFS looked more like consensus. Meanwhile the ML models are generally showing somewhat higher heights and less potential for a weakness at the surface and aloft over or near the Gulf of Mexico versus some GFS/ECMWF runs. ...Weather/Hazards Highlights... The most prominent focus for heavy rainfall during the period will be from the northern Plains into Upper Great Lakes during Friday- Saturday as upper dynamics emerging from western North America encourage northern Plains into Great Lakes/Ontario surface development. This system and associated fronts will interact with anomalous moisture that has been persisting over the region for multiple days. The Day 4 Excessive Rainfall Outlook for Friday maintains a Slight Risk area from parts of the Dakotas into the U.P. of Michigan (close to continuity) while the Day 5 ERO for Saturday proposes a Slight Risk area from northeastern Minnesota/northern Wisconsin into northern Michigan. Surrounding Marginal Risk areas account for less extreme but still locally significant rainfall potential over the northern Plains (Day 4) and over the Midwest along the trailing cold front (Day 5). Some areas across the northern tier will be sensitive to additional rainfall due to already wet ground heading into Friday. Elsewhere, the new Day 4 ERO maintains continuity with a Marginal Risk area over parts of the southern Rockies/Four Corners region due to the abundant moisture arriving from the east and some forecast instability, along with some model signals for locally enhanced rainfall. Guidance suggests that the greatest moisture anomalies should get pushed to the south and west during Saturday, with rainfall over the region likely to be lighter and more scattered--thus not meriting a risk area for Day 5. Meanwhile the Northeast continues to merit monitoring from Friday into the weekend. The combination of moisture and instability along with an east-west front settling over the region could support some locally enhanced rainfall rates and training/repeat activity. However antecedent conditions should be rather dry by that time, stream flows are already near to below normal, and model QPF is not exceptionally heavy thus far. Therefore the outlooks continue to depict no risk area at this time. Continued progression of the system that should be near the Great Lakes as of early Sunday will bring the trailing cold front through the eastern U.S. and trailing back into the Plains by the start of next week, bringing areas of showers and thunderstorms of varying intensity. The front reaching the Northwest this weekend may bring some light/scattered rainfall over northern areas. Continued progress of this front may generate some showers/thunderstorms over the central U.S. by the first half of next week. Some scattered diurnal convection may persist over the southern Rockies. Occasional showers/storms are possible along the Gulf Coast, but with increasing uncertainty over specifics of the Gulf of Mexico pattern at the surface and aloft by early next week keeping confidence low regarding any potential increase in moisture at that time. With a cold front reaching New England by Friday and providing a cooling trend there, the greatest temperature anomalies from late this week into the weekend should extend from the Midwest and Ohio Valley/Lower Great Lakes into the Mid-Atlantic and Northeast, with readings tending to be 10-15F above normal. This would translate to highs well into the 90s, along with lows in the upper 60s to upper 70s providing little overnight heat relief. Daily records for max/warm min temperatures will be possible within the above areas, up through warm mins early Monday (if they hold on through the calendar day). Meanwhile much of the West will trend warmer/hotter late this week into the weekend with highs reaching 10F or more above normal for a couple days or so. The front reaching the Northwest should bring cooler air to that region by Sunday-Monday, while some of the leading western heat should reach the northern- central High Plains at that time--connecting with lingering heat over parts of the Plains into southern half of the East. Clouds and rainfall will support below normal highs over the southern Rockies/High Plains late this week. 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 $$