Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 27 2024 Valid 12Z Tue Apr 30 2024 - 12Z Sat May 04 2024 ...Rounds of convection are likely in the central U.S. for much of next week... ...Overview... As the medium range period begins Tuesday, quasi-zonal upper flow will be in place over much of the lower 48, but embedded shortwaves could push frontal systems through the central and eastern U.S. and cause some showers and thunderstorms. Mean troughing should amplify over the Intermountain West Wednesday-Thursday and provide support for increasing convection over much of the central U.S., with heavy rain and flash flooding among the possible hazards. This convection will likely shift gradually eastward into late week. Precipitation including some higher elevation snow is possible across the Northwest into the northern/central Rockies, but with rather low confidence in the details at this time. ...Guidance/Predictability Assessment... While guidance runs since the 12Z/26 run appear to have resolved most of the prior divergence for the initial upper low energy (most likely comprising two separate centers) along the western U.S.-Canadian border, there is still a lot of spread for what becomes of the western half of the feature as well as upstream details. Recent trends, including from ECMWF-initialized machine learning models (MLs), support fairly amplified upper troughing over the West by Wednesday-Thursday. However from that point both dynamical guidance and the MLs diverge for timing and shape as the feature ejects from the West, affecting the surface evolution. The 06Z GFS appeared to be somewhat on the fast side of the spread but otherwise the envelope was broad enough to preclude discounting many solutions. 12Z model runs continue to differ by at least a day for timing of the upper system by next Friday, with corresponding effects on the Plains into Great Lakes surface development that most guidance is showing. Farther east the one item of note is that UKMET runs are notably aggressive with shortwave energy crossing New England around Thursday when consensus shows more upper ridging. There is still considerable spread for a compact upper low expected to reach western North America by Thursday. The new 12Z GFS/CMC runs maintain their theme from prior runs in showing a track into Washington state while the ECMWF is well north over British Columbia. The UKMET flipped from the GFS cluster in the 00Z run to shearing it out north of the ECMWF in the 12Z run. The GEFS/ECens means not surprisingly weaken the feature as it arrives but would imply a somewhat farther north track than the GFS cluster. The 00Z MLs were unanimous with a British Columbia track. Preference leans to the ECMWF/ML cluster. Interestingly the guidance gets a little closer together by Friday as the ECMWF/ML cluster drops the upper low southeast to around the Canadian border. What happens with this feature may ultimately have some influence on details of flow to the east. Significant differences continue as another upper trough (with possible low) approaches the Northwest. The 06Z GFS was on the eastern side of the spread in bringing a deep upper low into Washington state. Meanwhile the MLs on average show an intermediate idea between the deep and sometimes eastward GFS runs versus the slower/flatter ECMWF and ECens mean runs. The MLs are actually remarkably agreeable in showing the associated surface front somewhat offshore the Pacific Northwest as of early Saturday. As expected, GEFS/EC/CMC ensemble members show considerable spread for this and aforementioned features. In order to accommodate considerations for the various features of interest, the updated forecast started with a 00Z/06Z operational model composite early in the period. Then the blend eliminated the UKMET by Thursday, followed by splitting GFS/ECMWF input between consecutive 6-/12-hourly runs and incorporating 20-40 percent total of the 06Z GEFS/00Z ECens means. ...Weather/Hazards Highlights... On Tuesday, lessening upper-level support should lead to lower rainfall amounts and rainfall rates in the central U.S. compared earlier and later in the week. However, some convection is still likely to occur in a moist and unstable environment. Currently the best chance for rain with isolated flooding potential is across the south-central Plains to Mid-Mississippi Valley as a front stalls. Within that region there is some spread in location of locally heavy convective potential--GFS runs (including the new 12Z version) have been persistent in showing convection near the Red River while other models indicate local maxima farther north. A Day 4 Marginal Risk is in place in the ERO to cover these areas and will be refined with time. Some isolated moderate to locally heavy rain may be possible along/north of the Texas and Louisiana coast but what signal exists remains diffuse enough not to merit a risk area (maintaining the prior cycle's reduction that favored an earlier Marginal Risk area). By Wednesday, the upper flow (with troughing to the West and a southern stream shortwave) will become more supportive of more widespread heavy rain and thunderstorms for the central/southern Plains and Mid/Lower Mississippi Valley as moisture surges ahead of a wavy frontal system. Once again there is spread in the exact position of the heaviest totals, but rain amounts of 3-5 inches are likely, and some areas of heavy rain are likely to overlap with current/short range heavy rainfall locations. The Day 5/Wednesday ERO update incorporated latest guidance to yield only mild adjustments for the established Slight Risk area extending from parts of Texas into southern Iowa. Within this risk area, there may be somewhat greater potential between Kansas and Iowa given proximity to the stalling surface front. There is a little more spread farther south. This overall region will be sensitive due to wet antecedent conditions and potential for embedded upgrades will be possible depending on details of prior convection and how guidance consolidates for amounts/location of heaviest rainfall. Expect a slow trend of the rain/thunderstorms eastward through the rest of the week, corresponding to progression of the surface front. Elsewhere, one or more shortwaves and upper lows will lead to rounds of precipitation across the northern tier. On Tuesday, ample moisture with precipitable water values over the 90th or even 95th percentile will be in place across the northeastern U.S., with rain and thunderstorms likely. A shortwave aloft moving through (with associated wavy surface reflection) and some instability in place could lead to heavy rainfall causing isolated flooding concerns, so the Day 4/Thursday ERO maintains the previously introduced Marginal Risk area centered in upstate New York with its footprint stretching a bit into the northern Mid-Atlantic and western New England. Meanwhile the Northwest and into the northern and central Rockies could get rounds of precipitation through the week, including higher elevation snow. There is still some potential for one or more areas of locally enhanced totals but resolving specifics for important details of the supporting upper and surface pattern continues to be a challenge. A front nearing the Pacific Northwest coast may bring some precipitation to the region by next Saturday. Temperatures across the south-central U.S. all the way to the Eastern Seaboard will be warmer than average by generally 10-15F on Tuesday and Wednesday, while cooler than normal temperatures are likely for the northwestern quadrant of the lower 48. Then upper troughing and a cold front pushing from the Northwest should cool temperatures to near or below normal across the central U.S. by Thursday and reaching the east-central U.S. Friday and Saturday. Temperatures over the West may rebound to near normal or moderately above normal levels by late week, but confidence remains lower than average given continued guidance spread for upper flow details. Rausch/Tate 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