FXUS02 KWBC 191857 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Fri Jul 19 2024 Valid 12Z Mon Jul 22 2024 - 12Z Fri Jul 26 2024 ***Heat wave continues across the West, and showers/storms with heavy rainfall expected from the Four Corners to portions of the East*** 19Z Update: The 12Z model guidance suite remains in above average agreement on the synoptic scale through the end of next week, with a multi-deterministic model blend working well through Wednesday. The GFS does become more progressive with the trough over the Great Lakes region by Friday, and is also stronger with the Western U.S. ridge during that time. Some ensemble means were included by Wednesday night, and gradually increased to about 30-40% by the end of next week. The main uncertainties involve the timing and coverage of individual MCS events that will be the main drivers for heavy convective rainfall across the south-central U.S. next week. The NBM appeared to be both too light and also delayed with convective initiation across the Four Corners region with the monsoonal moisture in place, so more of the deterministic ECMWF and GFS was incorporated to boost expected QPF. The previous forecast discussion is appended below for reference. /Hamrick ------------------------ ...General Overview... Latest guidance shows an amplified pattern that will be fairly slow to evolve with time. A strong ridge extending from the Southwest U.S. into western Canada as of early Monday will produce dangerous and potentially record-setting heat into next week, with the Northwest seeing only gradual cooling as a Northeast Pacific upper low wobbles into western Canada. More persistent ridging over the Southwest will accompany monsoon conditions across the southern Rockies and vicinity. Meanwhile, a wavy front will be on the leading side of Great Lakes into southern Plains mean troughing aloft, leading to multiple days of rain/thunderstorms with areas of heavy rainfall from the southern Plains into the Mid-Atlantic and parts of New England. An area of tropical moisture will pass through Florida early next week to enhance daily shower and thunderstorm activity, followed by the Atlantic upper ridge building into the Southeast, peaking in strength around Wednesday- Thursday. ...Guidance/Predictability Assessment... Averages of latest dynamical guidance and machine learning (ML) models compare fairly well to each other for the western ridge through the period and Northeast Pacific upper low that reaches into British Columbia after midweek (along with the trough to the south). There are some detail differences with these features but these have low predictability several days out in time. Regarding the Great Lakes into southern Plains trough, there is reasonable agreement that energy dropping south from Canada by Tuesday-Wednesday should begin to eject an embedded weak upper low over the Midwest as of early Monday. Then some more noticeable differences arise with the overall trough after midweek. The 12Z ECMWF strayed to the slow/southwestern side of the spread with the core of its trough, while the GFS has been leaning a bit on the faster side among the remaining more progressive models/means. Interestingly, by next Friday most of the 12Z ECMWF-initialized ML models become fairly progressive as well, and with somewhat more amplitude than depicted in the latest GFS runs. There have been a couple cases in the past few months when the MLs ultimately were too amplified with forecast Northeast troughing so trends one way or the other will be worth monitoring. The new 00Z ECMWF has adjusted somewhat faster, now comparing better to the means and CMC by next Friday. By late next week the cluster that is faster than the 12Z ECMWF and less amplified than the MLs with the upper trough (with corresponding surface evolution/progression) represents the best intermediate solution. With the 12Z UKMET not comparing well to consensus for some details from the Upper Midwest northeastward after Monday, the first part of the forecast based on 12Z/18Z guidance incorporated the GFS/ECMWF/CMC. Then the latter half of the period gradually increased 18Z GEFS/12Z ECens weight, reaching 50 percent total by next Friday, while phasing out 12Z ECMWF input due to its questionably slow Midwest upper trough. ...Weather/Hazards Highlights... Expect dangerous heat over the West to extend into next week with high temperatures reaching the 90s and 100s and warm overnight lows providing only limited relief. The interior Northwest/far northern Rockies should see the greatest coverage of major to extreme Heat Risk values (along with daily record highs) on Monday, followed by gradual cooling moving in from the Pacific Northwest as an upper low eventually moving into British Columbia helps to lower heights aloft. Elsewhere temperatures in the West should remain well above average next week aside from perhaps some moderation over the Great Basin toward the end of next week. Some of the heat will extend into Montana/northern High Plains. The prolonged period of dry heat could also result in enhanced wildfire danger. Monsoonal moisture will linger over the Four Corners region and promote daily episodes of showers/thunderstorms. Isolated to scattered instances of flash flooding will be possible, especially near steep terrain and burn scars. The Days 4-5 Excessive Rainfall Outlooks covering Monday-Tuesday continued to show Marginal Risk areas, with the Day 4 outlook continuing a Slight Risk area over parts of northern New Mexico and south-central Colorado given sufficiently favorable guidance signals, persistence from the Day 3 time frame, and already wet ground conditions, along with vulnerable burn scar areas. A southern Plains into Northeast wavy front will provide a multi- day focus for areas of heavy showers and thunderstorms next week. The southern Plains part of the boundary may dissipate by midweek but another front pushing south from the northern tier should maintain a mean front over the eastern half of the country into late week. With ample moisture and instability in place, locally heavy rainfall will be possible and may cause instances of flash flooding, especially in urban and poor drainage areas where convection repeats/trains. During Day 4/Monday, guidance is signaling the best potential for heavy rainfall over parts of south-central/east-central Texas, and the existing Slight Risk area has been expanded some over this region. A broad surrounding Marginal Risk area remains in place from the southern Plains and central Gulf Coast to the Mid- Atlantic. The Day 5/Tuesday ERO continues a broad Marginal Risk area from the southern Plains/central Gulf Coast into the northern Mid-Atlantic/southern New England, with guidance suggesting that frontal waviness and associated moisture will lift northeastward over the eastern U.S. The southern portion of the existing Marginal across the Southeast and southern Mid-Atlantic was trimmed back some, while the northern portion was extended slightly more. There is also a Marginal Risk area on Tuesday over parts of the Upper Midwest where shortwave energy dropping south from Canada may produce locally heavy convection. 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 $$