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Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Thu Jun 17 2021 Valid 181200Z - 191200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL ILLINOIS EAST ACROSS INDIANA INTO EASTERN OHIO... ...SUMMARY... Scattered severe storms will be probable Friday across Illinois, Indiana, and Ohio. Severe gusts, possibly significant, will be capable of swaths of wind damage with the more intense storms. A couple of tornadoes are possible in addition to large hail. ...Synopsis... A mid-level anticyclone will reside across the southwest into the southern Great Plains on Friday. To its north, an elongated belt of strong mid-level flow will extend from the Great Lakes westward into the northern Great Plains early Friday morning. A mid-level shortwave trough will amplify during the period as it moves from the ND/Canadian border east-southeastward to the central Great Lakes late Friday night. At the surface, a front will become draped from the southern Great Lakes westward into the central High Plains, with a moistening air mass to the south over the mid MS Valley and OH Valley. Analysis of upper-air data on Thursday morning indicates that it is likely the area of richer low-level moisture (15-17 g/kg lowest 100 mb mean mixing ratio) on Thursday morning was sampled in between the raob network centered over the Ozark Plateau when compared to surface dewpoint observations. It is the uncertainty regarding the spatial extent and quality of this moisture pool and early day thunderstorm activity that could lead to consequential effects for this forecast across the OH Valley on Friday. ...Mid MS Valley through the OH Valley... Scattered showers/thunderstorms will probably be ongoing Friday morning across the southern Great Lakes where warm advection will be maximized. A warm front will advance east across much of OH during the day, and possibly into western PA and NY by early evening, as strongly veered 850 mb winds of 30-40 kt result in rapid airmass recovery. It is uncertain where the placement of a potential outflow boundary over OH may reside by afternoon. Models show a reservoir of mid 60s to lower 70s deg F dewpoints beneath an EML. Models vary appreciably on destabilization but it appears MLCAPE ranging from 2000-4000 J/kg is possible from east to west. Current thinking is a cluster of storms will develop during the afternoon across IN/OH and spread southeast. Although 850 mb flow will be veered, the presence of the aforementioned outflow and its enhancement in low-level shear may conditionally focus a tornado risk. Organized storms including supercells and bowing segments are probable. Weaker upper-level flow will favor a transition to HP supercells and into QLCS/bowing segments with a wind hazard. A separate cluster of development seems likely to develop near the front towards evening over the mid MS Valley and it spreading east-southeast. Severe gusts, potentially significant in localized swaths, will be the primary threats with this activity. ...Central High Plains... Model guidance shows upslope flow into northeast CO beneath 15-30 kt westerly 500 mb flow on the southern periphery of the stronger westerlies. Isolated to scattered storms are forecast during the afternoon. Strong heating and steep lapse rates may yield a few strong storms capable of an isolated hail/wind threat before this activity weakens during the evening. ...Coastal LA... A tropical depression is forecast to develop and move north across the western Gulf of Mexico, approaching the TX/LA coastal region early on Saturday. Coincident with the low, wind fields and shear will also increase, along with tropical moisture aiding instability. A substantial amount of convection is possible over LA, and a couple tornadoes will conditionally be possible wherever mid 70s dewpoints can reach land. Due to uncertainty in speed, there is some chance that the better air mass remains just offshore for this forecast period. ..Smith.. 06/17/2021Read more
Updated 17 June 2021 | 12:37 pm
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1130 AM CDT Thu Jun 17 2021 Valid 171630Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHEAST MN...SOUTHERN WI...NORTHERN PORTIONS OF IA/IL... ...SUMMARY... Severe storms are expected late this afternoon into tonight across parts of the Upper Mississippi Valley, from central/southern Wisconsin and southern Minnesota into northern portions of Iowa and Illinois. A few tornadoes, damaging wind gusts exceeding 75 mph, and hail up to the size of baseballs will be possible. ...Upper MS Valley... Primary change from prior outlook is to expand greater probabilities farther north/east across parts of WI and IL, with the expectation of the wind threat persisting longer in this region tonight. A mid-level impulse/speed max over SD, responsible for morning ACCAS across the eastern portion of the state, will aid in ascent along a southeast-moving cold front. Thunderstorm initiation is expected across west-central WI into southeast MN as MLCIN wanes near the front during the late afternoon. Activity should eventually backbuild southwestward into northeast/north-central IA this evening, but uncertainty is greater with how far into central IA convection will ultimately become sustained owing to greater influence of the EML and persistence of a relatively hot/dry boundary-layer across KS/southern NE into southern IA. While guidance will probably be somewhat overdone with the degree of evapo-transpiration-induced moistening ahead of the front based on 12Z morning soundings and ongoing drought, large buoyancy with MLCAPE of 2500-3500 J/kg should develop within a confined corridor given steep mid-level lapse rates. The mid-level speed max (around 60 kts at 500 mb) will yield strong effective shear, especially with northern extent. This suggests initial supercells will have the potential to produce very large hail despite the relatively high freezing levels. Enlarging low-level hodographs as a southwesterly low-level jet strengthens this evening should maximize the supercell tornado threat between 22-02Z. Storms are expected to predominately coalesce and grow upscale into one or more bowing clusters with the potential to produce significant severe wind gusts (greater than 75 mph) and brief tornadoes this evening. The damaging wind threat may linger into the early overnight as strong low to deep-layer shear attempts to compensate for increasingly pronounced MLCIN. ...Ozarks to Lower Red River Valley.. A remnant MCV over southwest MO will drift southward across western AR this afternoon. Strong surface heating where dew points remain from 70-75 F will support large MLCAPE over 3000 J/kg. Isolated to perhaps scattered thunderstorms should develop southwest from southern MO towards the AR/OK/TX border area. A belt of 20-30 kt mid-level northeasterlies may provide adequate shear to support a couple loosely organized multicell clusters. Isolated damaging downbursts along with severe hail are possible during the late afternoon and early evening. ..Grams/Squitieri.. 06/17/2021Read more
Updated 17 June 2021 | 11:38 am
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0739 AM CDT Thu Jun 17 2021 Valid 171300Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO TONIGHT FOR PARTS OF THE UPPER MS VALLEY... ...SUMMARY... Severe storms are expected late this afternoon into tonight for parts of the upper Mississippi Valley, from northern Iowa/southern Minnesota into southern Wisconsin and northern Illinois. Wind gusts near or exceeding 75 mph, hail up to the size of baseballs, and a couple of tornadoes will be possible. ...Upper MS Valley this afternoon into tonight... A midlevel shortwave trough over the northern Plains/southern Prairie Provinces has crested the ridge, and heights will be suppressed east-southeastward toward the upper MS Valley and upper Great lakes by tonight. An associated, weak surface cold front/trough will likewise move slowly southeastward across NE/IA/MN/WI through tonight. The front and any lingering outflow boundaries from overnight convection will likely focus afternoon and evening thunderstorm development across southeast MN and along the IA/MN border, in phase with an embedded speed max moving eastward from SD (as evidence by the weak midlevel convection over SD this morning). The thermodynamic environment will feature steep midlevel lapse rates with a warm elevated mixed layer extending eastward from the Rockies, atop boundary-layer dewpoints in the 65-70 F range. Strong surface heating in the wake of morning elevated convection will drive large buoyancy (MLCAPE in excess of 4000 J/kg) and weakening convective inhibition as surface temperatures warm into the 90s. Effective bulk shear of 40-50 kt and the large CAPE will support a threat for supercells initially, with very large hail (up to baseball size) and damaging winds. Any semi-discrete storms in the zone of stronger low-level shear/warm advection closer to the MS River will also pose a threat for a couple of tornadoes. Otherwise, storms are expected to coalesce and grow upscale into one or more bowing clusters with the potential to produce significant damaging winds (greater than 75 mph) prior to the convection spreading east of the larger buoyancy late tonight. ...AR this afternoon/evening... A remnant MCV over west central MO this morning will continue southward over AR by later this afternoon. Strong surface heating (temperatures warming into the lower 90s) and boundary-layer dewpoints in the lower 70s will support MLCAPE in excess of 3000 J/kg, and isolated to widely scattered thunderstorms are expected to form by mid-late afternoon. Deep-layer, north-northeasterly shear of 20-30 kt and DCAPE in excess of 1200 J/kg suggest that any cells/small clusters will move toward the south-southwest with the potential to produce isolated damaging downbursts and large hail for a few hours this late afternoon/evening. ..Thompson/Leitman.. 06/17/2021Read more
Updated 17 June 2021 | 7:48 am