• Storm Prediction Center 20 08

    From LU9DCE@21:5/101 to )@WW on Tue Aug 20 19:45:21 2019
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    WW 601 SEVERE TSTM IA MN SD 200455Z - 201200Z
    URGENT - IMMEDIATE BROADCAST REQUESTED Severe Thunderstorm Watch
    Number 601 NWS Storm Prediction Center Norman OK 1155 PM CDT Mon
    Aug 19 2019
    The NWS Storm Prediction Center has issued a
    * Severe Thunderstorm Watch for portions of Western and central
    Iowa Far southwest Minnesota Far southeast South Dakota
    * Effective this Monday night and Tuesday morning from 1155 PM
    until 700 AM CDT.
    * Primary threats include... Scattered damaging winds likely
    with isolated significant gusts to 75 mph possible Scattered large
    hail and isolated very large hail events to 2.5 inches in diameter
    possible
    SUMMARY...Multiple elevated supercells will likely develop through
    the early morning hours before growing upscale into a linear cluster
    that may eventually bow across central to south-central Iowa.
    The severe thunderstorm watch area is approximately along and 75
    statute miles east and west of a line from 45 miles east of Lamoni
    IA to 15 miles west northwest of Worthington MN. For a complete
    depiction of the watch see the associated watch outline update
    (WOUS64 KWNS WOU1).
    PRECAUTIONARY/PREPAREDNESS ACTIONS...
    REMEMBER...A Severe Thunderstorm Watch means conditions are
    favorable for severe thunderstorms in and close to the watch area.
    Persons in these areas should be on the lookout for threatening
    weather conditions and listen for later statements and possible
    warnings. Severe thunderstorms can and occasionally do produce
    tornadoes.


    AVIATION...A few severe thunderstorms with hail surface and aloft
    to 2.5 inches. Extreme turbulence and surface wind gusts to 65
    knots. A few cumulonimbi with maximum tops to 550. Mean storm motion
    vector 33030.
    ...Grams
    WW 0601 Status Updates
    STATUS FOR WATCH 0601 HAS NOT BEEN ISSUED YET
    SPC 1200Z Day 1 Outlook
    Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0102
    AM CDT Tue Aug 20 2019
    Valid 201200Z - 211200Z
    ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
    OF THE MIDDLE MS VALLEY...
    ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
    MN...
    ...SUMMARY... An organized cluster of thunderstorms capable of
    severe wind gusts and hail will overspread portions of the middle
    Mississippi and lower Ohio Valleys today. Hail and isolated strong
    winds are also possible across portions of Minnesota this morning.
    ...Mid MS Valley eastward across the OH Valley... An organized MCS
    will likely be moving through south-central/southeast IA early this
    morning. Expectation is for this MCS to be near its mature phase
    at this point with a well-developed rear-inflow jet and strong
    cold pool resulting in strong wind gusts. A few of these gusts
    could be greater than 75 mph. As the MCS continues southeastward
    through northwest MO and central IL later this morning and into
    the early afternoon, it is expected to gradually lose intensity
    as its cold pool accelerates and loses depth. By the time the MCS
    reaches IN and western KY, it is expected to be characterized by
    weakly organized multicells along the cold pool. Even with this
    anticipated weakening, strong wind gusts are still possible.
    ...Central High Plains/Central Plains... A cold front is
    expected to push into the Upper Midwest and central Plains,
    likely extending from western WI southwestward into northeastern
    CO during the late afternoon. Moist upslope flow is anticipated
    into eastern WY/northeast CO behind this front, with the
    resulting convergence along the lee trough expected to result in
    thunderstorm initiation. Downstream environment characterized by
    strong buoyancy and at least moderate vertical shear will support
    supercell storm structures capable of very large hail and strong
    wind gusts. These storms are expected to continue eastward into
    NE during the evening. A strengthening low-level jet will lead to
    a reinvigoration of any ongoing storms and/or development of new
    storms, primarily across central and eastern NE. Given the presence
    of ample low-level moisture and steep mid-level lapse rates, large
    hail is probable with these storms. Isolated very large hail (i.e.
    greater than 2" in diameter) is possible. Dry mid-levels could also
    contribute to strong wind gusts.
    ...Upper Midwest... Ample low-level moisture and steep mid-level
    lapse rates will result in strong buoyancy ahead of the approaching
    cold front this morning across northern/central MN. Stable low-levels
    (below 850 mb) will likely limit storm strength but large hail and
    occasional strong gusts through the stable layer are possible.
    ...Northern Mid-Atlantic... Showers and thunderstorms will
    likely develop during the afternoon within the moist and diurnally
    destabilized air mass across the region. Vertical shear will be
    weak, leading to predominately disorganized, outflow-dominant
    multicells. Even so, the very moist air mass and steep low-level
    lapse rates support water-loaded downbursts capable of damaging
    wind gusts.
    ..Mosier/Nauslar.. 08/20/2019
    SPC 0600Z Day 2 Outlook
    Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1254
    AM CDT Tue Aug 20 2019
    Valid 211200Z - 221200Z
    ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
    THE NORTHEAST...
    ...SUMMARY... Scattered storms, some severe, are possible Wednesday
    over parts of the Northeast, and over much of the central Plains
    into the mid Mississippi Valley. Damaging wind gusts and marginal
    hail are possible in both regions.
    ...Synopsis... An upper low will move from Ontario into Quebec,
    with a leading shortwave trough moving across the Great Lakes during
    the day and New England overnight. A cold front will stretch from
    southwest Ontario to IL by late afternoon, with mid to upper 60s F
    dewpoints spreading into the Northeast. Aiding moisture advection
    as well as low-level shear will be southwesterly 850 mb winds
    increasing to 25-30 kt, while mid to upper level winds increase
    throughout the period.
    To the west, a convectively reinforced boundary is expected across
    northern MO, KS, and into eastern CO, with a very moist and unstable
    air mass to the south. Bouts of thunderstorms are likely throughout
    the period near this boundary, with the greatest concentration over
    MO, closer to the Great Lakes trough influence.
    ...Northeastern states... Heating will lead to numerous
    thunderstorms across the moist sector by 18Z over much of
    NY and northeast PA, expanding across New England during the
    afternoon. Around 30-35 kt of deep-layer shear and 100-150 m2/s2
    effective SRH will be sufficient to sustain severe storms, including
    isolated supercells capable of marginal hail, a brief/weak tornado,
    or damaging wind. Due to the expected large number of storms,
    mergers are likely, and a few line segments may form as well. Storms
    may persist well into the evening and overnight into northern New
    England when the stronger height falls occur.
    ...Central Plains into the Mid MS and lower OH Valley... Storms are
    expected to be ongoing from NE across MO and into IL Wednesday
    morning along an east-west oriented front, and where substantial
    instability will exist. This outflow-reinforced front will likely
    translate south during the day, with continuous areas of storms
    over KS, MO, and IL. Winds aloft will generally be westerly and
    below 20 kt, thus disorganized storms are most likely. However,
    a few small bows are possible, with localized areas of wind damage.
    Given such strong instability, the strongest storms may produce hail.
    To the west, east/northeast surface winds will bring a moist air
    mass into east-central WY and CO, where capping will be a concern.
    Storms are likely to form over the higher terrain, with a narrow
    north-south zone of hail or wind threat. Other storms are likely
    farther south in the well-mixed and hot air, from northeast NM
    across northern OK and AR, with localized downbursts possible.
    ...MAXIMUM RISK BY HAZARD... Tornado: 2% - Marginal Wind:
    15% - Slight Hail: 5% - Marginal
    ..Jewell.. 08/20/2019
    SPC Day 1 Fire Weather Outlook
    Day 1 Fire Weather Outlook NWS Storm Prediction Center Norman OK
    0136 AM CDT Tue Aug 20 2019
    Valid 201200Z - 211200Z
    ...NO CRITICAL AREAS...
    ...Synopsis... An upper-level ridge will continue to amplify over
    the western CONUS into western Canada today as an upper-level
    shortwave trough approaches the Pacific Northwest. Subtropical
    moisture will begin to move northward into the Southwest and
    eastward into southwest Montana from the northern Plains. A surface
    cold front will push southward up against the eastern slopes of the northern/central Rockies with hot/dry/unstable conditions prevailing
    across much of the Intermountain West.
    Isolated dry thunderstorms may form across southwest Montana
    and northwest Wyoming this afternoon/evening. However, given
    the uncertainty regarding coverage and marginal fuel conditions
    where storms are currently forecast to develop, an isolated dry
    thunderstorm area was not included.
    ..Nauslar.. 08/20/2019
    ...Please see www.spc.noaa.gov/fire for graphic product...
    SPC Day 2 Fire Weather Outlook
    Day 2 Fire Weather Outlook NWS Storm Prediction Center Norman OK
    0142 AM CDT Tue Aug 20 2019
    Valid 211200Z - 221200Z
    ...NO CRITICAL AREAS...
    ...Synopsis... An upper-level shortwave trough is forecast to
    move into the Pacific Northwest and northern Rockies tomorrow. The
    trough will likely deepen and stretch through northern California and
    much of the Great Basin. An associated surface cold front will move
    eastward across the Pacific Northwest and into northern California,
    the northern Great Basin, and the northern Rockies.
    Dry/breezy conditions will likely develop across portions of the
    Columbia Basin/Gorge and into central/eastern Oregon ahead of the
    approaching cold front. Elevated/locally critical conditions may
    develop, but the window for these conditions should be relatively
    short-lived and followed by cool/moist conditions. Thunderstorms
    may also develop along/ahead of this cold front across eastern
    Washington into the Idaho Panhandle and western Montana. However,
    given the higher precipitable water values (0.8-1.2") along/just
    ahead of the cold front and the cool/moist airmass behind the front,
    an isolated dry thunderstorm area was not included.
    ..Nauslar.. 08/20/2019
    ...Please see www.spc.noaa.gov/fire for graphic product...

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