• Storm Prediction Center 09 08

    From LU9DCE@21:5/101 to ALERT on Fri Aug 9 08:10:01 2019
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    SPC 1200Z Day 1 Outlook
    Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257
    AM CDT Fri Aug 09 2019
    Valid 091200Z - 101200Z
    ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER CENTRAL
    SOUTH DAKOTA AND FAR NORTH-CENTRAL NEBRASKA...AND CENTRAL OREGON... ...SUMMARY... Severe storms are likely over parts of the Northern
    Plains and over parts of Oregon. A few strong to severe storms are
    also possible over parts of the Mid Atlantic and central High Plains. ...Synopsis... An upper high will remain centered over the southern
    Plains, with upper troughs over the West Coast and from the Great
    Lakes into Northeast. A surface low will develop from SD southward
    across the High Plains, with a stationary front extending east
    from the SD low across the MO and OH Valleys. A moist air mass
    will remain over much of the Plains, and will result in unstable
    conditions especially over SD and NE where minor disturbances will
    round the upper ridge, enhancing shear and lift. Farther east, the
    primary synoptic boundary will move east as a cold front across VA
    and the Delmarva, providing a focus for storms.
    To the west, cooling aloft will aid destabilization over OR and
    western ID, with increasing winds aloft aiding in severe potential
    during the afternoon and evening.
    ...Northern Plains... A few storms are possible early in the day
    moving from west to east across SD in a zone of low-level warm
    advection with 30 kt southerly winds at 850 mb. In the wake of this
    activity, strong heating will occur across WY and western NE, with
    the low-level lapse rate plume extending into southwest SD. Heating
    combined with a plume of mid 60s F dewpoints will lead to areas of
    strong instability, beneath modest westerly midlevel winds. Storms
    are expected to initiate by 21Z along a line from west-central
    SD into parts of the NE Panhandle, and few could be supercells.
    Lapse rates aloft will not be particularly steep due to warming
    aloft but large hail is still likely given cellular storm mode. A
    brief tornado or two cannot be ruled out as effective SRH increases
    to 200-400 m2/s2 east of where storms initiate.
    ...Oregon... Strong heating beneath cooling profiles aloft will lead
    to very steep lapse rates through a deep layer, while winds aloft
    also increase with the approaching upper trough. Storms are expected
    to initiate over southern OR by afternoon in a zone of increasing
    differential divergence, and will spread northward across the state,
    and possibly into western ID as well. A few storms may contain small
    hail but the main threat is expected to be damaging outflow winds.
    ...Mid Atlantic... Relatively strong northwest flow aloft
    will remain over the region with the main upper trough over the
    Northeast. Strong heating will occur ahead of the cold front, with
    storms initiating after 18Z over central VA, spreading southeastward
    during the afternoon. Although storm coverage may remain isolated,
    any storms will have both hail and wind potential given favorable
    time of day, and sufficient deep-layer shear to sustain storms for
    several hours.
    ..Jewell/Cook.. 08/09/2019
    SPC 0600Z Day 2 Outlook
    Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 0100
    AM CDT Fri Aug 09 2019
    Valid 101200Z - 111200Z
    ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION
    OF THE NORTHERN ROCKIES...
    ...SUMMARY... Isolated strong to severe storms are expected
    Saturday afternoon and evening mainly over the northern Rockies
    with large hail and damaging wind the main threats. Other strong to
    severe storms are possible over the central High Plains and coastal
    North Carolina.
    ...Synopsis...
    An omega blocking pattern will prevail over the U.S. Saturday
    with an upper ridge situated over the southern Plains flanked by
    upper troughs over the western and eastern states. At the surface
    a cold front should extend from a weak surface low over eastern SD
    southwest into northwest NE and move slowly southeast during the
    day. A quasi-stationary front will extend southeast from the low
    through the middle MS and into the TN Valley regions. Farther north
    another cold front will move southward into the northern Plains. A quasi-stationary front will reside across the eastern Carolinas.
    ...Northern Rockies...
    Steep mid-level lapse rates will reside over the northern Rockies
    with a belt of stronger southwesterly winds aloft downstream from an approaching upper trough. Diabatic heating over the higher terrain
    should promote moderate instability with 1000-1500 J/kg MLCAPE
    possible during the afternoon and evening. Storms are expected to
    develop over the mountains and within corridor of deeper ascent
    associated with the upper trough. Effective bulk shear from 35-40 kt
    will support some organized structures including a few supercells
    with isolated large hail and damaging wind possible from late
    afternoon through mid evening.
    ...Central High Plains...
    An easterly upslope component is expected over a portion of the
    central Rockies Saturday afternoon, and storms are expected to
    develop over the higher terrain and spread east into the High
    Plains where dewpoints in the upper 50s to low 60s F will promote
    moderate instability (2000-2500 J/kg MLCAPE). Potential will exist
    for vorticity maxima to move along northern periphery of upper ridge
    and into the central High Plains. A risk for hail and downburst winds
    may accompany the storms as they move into the High Plains. However,
    vertical wind profiles and storm coverage will be modulated by
    the timing and strength of these impulses which remains somewhat
    uncertain at this time. Will therefore introduce a MRGL risk this
    update will possible SLGT risk in later outlooks.
    ...Coastal North Carolina...
    Storms may develop across the eastern Carolinas along and ahead
    of the aforementioned front where richer low-level moisture will
    support moderate instability. This region will also reside within
    belt of 30-40 kt mid-level winds within base of an upper trough.
    This environment will be conditionally favorable for a few strong
    to severe storms during the afternoon and have therefore introduced
    a MRGL risk category this update.
    ...MAXIMUM RISK BY HAZARD... Tornado: 2% - Marginal Wind:
    15% - Slight Hail: 15% - Slight
    ..Dial.. 08/09/2019
    SPC Day 1 Fire Weather Outlook
    Day 1 Fire Weather Outlook NWS Storm Prediction Center Norman OK
    0104 AM CDT Fri Aug 09 2019
    Valid 091200Z - 101200Z
    ...Synopsis... A longwave trough centered just west of the northern
    California coast will continue its slow trek eastward toward the
    West Coast throughout the day. As this occurs, mid-level flow will
    increase in a corridor from southern California northward to Idaho.
    The deep southerly flow will maintain moist mid-level profiles in
    much of the western third of the CONUS, and lift associated with the approaching trough and surface heating should foster several areas
    of thunderstorms. Additionally, a favorable pressure gradient and terrain-related forcing should foster areas of windy conditions
    across Nevada and vicinity through the evening.
    ...Central Nevada northward into Washington for thunderstorms...
    The aforementioned synoptic pattern will foster scattered areas of thunderstorms as early as mid-day while gradually increasing in
    coverage and intensity through the afternoon and evening hours.
    These storms should provide wetting rains to several areas,
    with isolated dry lightning strikes occurring on the periphery of
    wetting thunderstorm cores. Coverage will increase with northward
    extent into Oregon and southwestern Idaho throughout the day,
    with widespread areas of gusty outflow winds amidst areas of
    dry fuels suggestive of erratic fire behavior and complicated
    suppression efforts. A few of these wind gusts may exceed severe
    limits - refer to SPC Day 1 Convective Outlooks for more updates on
    this threat. At least pockets of dry fuels will exist as far east
    as eastern Idaho, and isolated dry thunderstorm potential has been
    highlighted in this area with storms expected through early evening.
    Storms will continue to lift northward and persist in many areas
    of Washington and Oregon overnight.
    ...Much of Nevada and far northeast California for elevated fire
    weather conditions... Surface troughing and vertical mixing
    processes will foster areas of 20-25 mph surface winds that will
    be strongest in a couple of corridors - one in the immediate
    lee of the Sierras and another along an axis from southeastern
    into east-central Nevada. Areas of critically low RH values will
    also exist for a few hours during peak heating, though clouds and
    areas of precipitation may hinder the degree of mixing/lowered
    RH in several areas as well. The overall scenario appears to be
    consistent with elevated fire weather thresholds, though locally
    critical fire weather will probably develop over southeastern Nevada
    for at least an hour or two.
    ..Cook.. 08/09/2019
    ...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
    0126 AM CDT Fri Aug 09 2019
    Valid 101200Z - 111200Z
    ...Synopsis... The mid-level trough responsible for D1 fire
    weather concerns in the West will take on a negative tilt while
    slowly migrating northeastward over Oregon and vicinity. As this
    occurs, a belt of enhanced mid-level flow will shift eastward toward
    eastern Nevada and eastern Idaho. Meanwhile, areas of scattered
    to widespread thunderstorm activity will once again migrate over
    inland portions of the Pacific Northwest and northern California.
    ...Eastern Nevada, far western Utah, and eastern Idaho...
    Vertical mixing processes beneath the mid-level jet core will
    encourage development of a belt of relatively strong (15-25 mph) south-southwesterly surface winds during peak heating periods across
    the region. This region also resides on the western fringe of much
    of the rainfall over the past month or so, with dry fuels supporting
    ongoing fires across the region. One lingering uncertainty with
    regard to the extent of the fire-weather threat in this area will
    be potential for wetting rainfall over the region on D1/Fri, which
    may temper fuels in a few areas. However, elevated to locally
    critical fire weather will likely exist given near-critical RH
    values expected during the afternoon. Some portions of this region
    may need a critical upgrade in later outlook updates.
    ...Washington, Oregon, and northern California for thunderstorms...
    The continued presence of mid-level moisture and a nearby trough
    will again foster areas of northeastward-moving clusters of storms
    across the region. However, the extent of dry thunder potential is
    a bit less clear compared to prior days due to 1) expected areas
    of wetting rainfall on D1/Fri, which should temper fuel states
    on at least a localized basis and 2) the coverage of wetting
    rainfall during the afternoon and evening, which should further
    temper fuel states over a much broader area especially in Oregon.
    CAMS/high-res guidance even depicts upscale growth into linear
    complex(es) during peak heating hours. Nevertheless, areas that
    can remain dry will be susceptible to isolated dry lightning strikes
    outside of heavier thunderstorm cores, and gusty/erratic winds near
    downdrafts are expected.
    ..Cook.. 08/09/2019
    ...Please see www.spc.noaa.gov/fire for graphic product...

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