Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1254 AM CDT Mon Sep 18 2017 Valid 191200Z - 201200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHEASTERN NORTH DAKOTA INTO EASTERN SOUTH DAKOTA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED RISK AREA ACROSS THE EASTERN DAKOTAS...NORTHEASTERN NEBRASKA...AND WESTERN WISCONSIN... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK AREA FROM THE CENTRAL DAKOTAS SOUTHWARD TO SOUTHERN NEBRASKA AND EASTWARD TO WESTERN WISCONSIN... ...SUMMARY... An appreciable severe threat will exist in portions of the northern Great Plains Tuesday evening, with a lesser threat shifting toward the Upper Mississippi Valley early Wednesday. Damaging wind gusts are the primary threat, although hail and a few tornadoes cannot be completely ruled out. ...Synopsis... Broadly cyclonic mid-level flow will persist across the West and central/northern Great Plains Tuesday. A vigorous disturbance will eject from the northern Rockies toward the Dakotas while taking on a negative tilt. As this occurs, strengthening mid-level flow will overspread portions of the Dakotas and Minnesota throughout the forecast period. Weak and zonally oriented flow will exist across the Southeast, while the circulation associated with TC Jose approaches portions of New England (see National Hurricane Center forecasts for more information on Jose). At the surface, a north-to-south oriented lee trough across the western Dakotas will eventually merge with an accelerating cold front while shifting eastward throughout the afternoon and evening. Ahead of the trough/front, a moist axis (characterized by upper 50s to upper 60s dewpoints) will be maintained by increasing southerly low-level flow. The synoptic setup will foster an increasing severe threat into the evening and overnight hours. ...Dakotas/eastern Nebraska eastward to western Wisconsin... Insolation will foster a destabilizing, although strongly capped, airmass along an axis from central/eastern North Dakota south-southeastward into Nebraska/Iowa. Forcing associated with the advancing mid-level wave and increasing surface convergence along the front will foster increasing convective development from mid/late afternoon onward -- initially in central North Dakota and extending southward perhaps into central Nebraska. These storms should organize fairly quickly into a linear, forward propagating complex due to the strong forcing, with moderate to strong buoyancy and strengthening low-level and deep shear profiles fostering an enhanced damaging wind threat (particularly in the Slight and Enhanced areas). Instances of hail are also possible, and tornadoes cannot be completely ruled out given strong low-level shear profiles and strongly curved low-level hodographs. These storms will become increasingly elevated with time as they spread eastward into eastern Minnesota and western Wisconsin overnight, although steep mid-level lapse rates will foster an isolated severe hail threat there through 12Z Wednesday. ..Cook.. 09/18/2017Read more
from SPC Forecast Products http://ift.tt/GB0aDL
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