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Archives for SPC Convective outlook are updated daily with a live map a the beginning of the article. Touch or click map to get possible update, and follow link at end of article for referral to NOAA/NWS Storm Prediction Center. Archives for Chicago's hourly weather data were updated 24-48 hours after end of each day prior to Dec. 2020.

Monday, July 25, 2022

SPC Jul 25, 2022 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Mon Jul 25 2022 Valid 251300Z - 261200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN MID-ATLANTIC AND NEW ENGLAND... ...SUMMARY... Scattered strong-severe thunderstorm gusts are possible across parts of the northern Mid-Atlantic and New England. ...Synopsis... In mid/upper levels, northern-stream flow is becoming characterized by broadly cyclonic character across the northern Rockies to the Great Lakes and Northeast, with the Great Lakes part being nearly zonal. This is in response largely to the presence of two dominant cyclones over Canada: 1. A long-lived gyre over the James Bay region, forecast to move eastward across northern QC. A shortwave trough over its southern sector is apparent in moisture-channel imagery from the northern ON/QC border across northern Lake Huron and portions of Lower MI. This trough will pivot across southern QC and the St. Lawrence Valley region today, with its southern fringe glancing the northern NY/northern New England area. A weaker, trailing vorticity lobe will move eastward from Lake Erie across NY this afternoon/evening, then weaken as it crosses New England. 2. A strengthening vortex digging southeastward from the SK/NWT border region across northern MB. Heights will fall across the Dakotas late in the period (overnight) as a basal shortwave trough approaches. In advance of that, a small shortwave trough -- initially apparent over southeastern MT/northeastern WY -- will move east-southeastward to central SD today, while weakening, then across southern MN overnight. At 11Z, the surface analysis showed a cold front from western NY across central OH, the lower Ohio River Valley, and MO Ozarks, becoming a warm to stationary front across southern KS to a low between LBL-GAG. By 00Z, the cold front should reach eastern New England, southern PA, eastern KY, western TN, then become a wavy/ quasistationary front over southwestern MO and southern KS. By 12Z, the front should extend across NJ to eastern KY, then quasistationary near its previous position across MO/KS. A separate cold front -- related to the trailing northern-stream cyclone over central Canada -- will move southeastward across the northern High Plains overnight, reaching from eastern ND to central WY by 12Z tomorrow. ...New England and northern Mid-Atlantic region... Scattered thunderstorms are expected to form through this afternoon along/ahead of the surface cold front, near a prefrontal surface trough, and farther south over portions of the Virginias, over strongly heated higher terrain in the warm sector. An existing plume of clouds/precip and widely scattered embedded thunderstorms, from northern ME into PA, should continue to break up gradually on the north end this morning, permitting some destabilization in its wake, but also setting up localized to mesobeta-scale areas of differential heating that may aid storm initiation from midday into the afternoon as well. Scattered damaging gusts, and several severe/50-kt gusts, are possible with the midday/afternoon activity. A marginal/conditional tornado threat also may exist over northern areas, where low-level and deep shear will be the greatest under relatively maximized flow aloft. Instability generally will increase with southward/southeastward extent away from the early cloud cover, as well as between it and the front, under cooler air aloft. Meanwhile, boundary-layer moisture will remain favorable across the entire swath (i.e., surface dewpoints upper 60s to low 70s F). This will offset modest midlevel lapse rates to foster a plume of MLCAPE around 1500-2000 J/kg from northern VA and the Chesapeake Bay area, narrowing northeastward to the southern/eastern coastal areas of New England away from marine-layer influences. MLCAPE should diminish northwestward from there, but still may reach 500-1200 J/kg over ME, except for Downeast areas with onshore flow. Forecast effective- shear magnitudes range from a supercell-favoring 40-50 kt over northern New England to around 20-30 kt or less across central VA to eastern WV, though a more strongly-heated, deeply mixed boundary layer may develop in southern areas in support of pulse/multicell gust potential. ...Mid Mississippi Valley region overnight... Given some capping and lack of large-scale support aloft, considerable uncertainties exist as to whether (and if so, how much) convection can develop along or just north of the front this afternoon. Still, as the southwesterly low-level jet intensifies into the region north of the Missouri River this evening, warm advection and isentropic lift should strengthen along the elevated frontal surface until LFC is reached. With or without antecedent/ diurnal activity moving into western parts of the area, thunderstorms should develop near the nose of the LLJ where ascent will be focused strongest. Forecast MUCAPE around 1000-1500 J/kg, amidst 35-40-kt effective-shear magnitudes and 200-300 J/kg effective SRH (accounting for elevated inflow) suggest a few organized cells are possible, particularly early in the convective cycle before a more front-parallel linear configuration evolves. Potential upscale growth into an MCS and eastward/forward propagation across IL is too uncertain to account for in unconditional probabilities at this stage. ...Central/northern High Plains... Conditional severe potential (hail/wind) is evident in this region this afternoon into this evening, with a deeply mixed diurnal boundary layer, steep low/middle-level lapse rates, and pockets of favorable/residual moisture all expected to support a plume of 1500-2000 J/kg MLCAPE from parts of western SD into the NE Panhandle. Any convection that does develop will be in an environment of modest-magnitude but strongly veering low-level winds with height, leading to long hodographs and around 40-50 kt effective-shear magnitudes. However, convective guidance is quite inconsistent with timing, location and even existence of development in this region, due in part to the dearth of large-scale support related to its position behind the aforementioned shortwave trough moving out of southeastern MT. Surface forcing also appears nebulous east of a lee trough in eastern parts of WY/MT. Still, if convective guidance and mesoscale trends concentrate convective focus better than apparent for the last outlook cycle (or this one), an area of unconditional severe probabilities may need to be reintroduced at some point today. ..Edwards/Gleason.. 07/25/2022 Read more CHECK UPDATE ZOOM GRAPHIC
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