SPC Jan 20, 2018 2000 UTC Day 1 Convective Outlook

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Jan 20, 2018 2000 UTC Day 1 Convective Outlook
Updated: Sat Jan 20 19:27:37 UTC 2018 (Print Version | 20180120 2000Z Day 1 shapefile | 20180120 2000Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
 Forecast Discussion
 SPC AC 201927 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0127 PM CST Sat Jan 20 2018 Valid 202000Z - 211200Z ...NO THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorms appear unlikely through tonight across the contiguous United States. ...Discussion... No changes to 1630z outlook. ..Darrow.. 01/20/2018 .PREV DISCUSSION... /ISSUED 1028 AM CST Sat Jan 20 2018/ ...Synopsis and Discussion... A vigorous southern-stream shortwave trough will move across the Southwest and Great Basin to the Four Corners by early Sunday morning as a Pacific cold front likewise moves eastward across these same areas. Strong forcing for ascent will modestly steepen mid-level lapse rates preceding this shortwave trough across parts of AZ/UT into western CO/NM this afternoon and evening. Some mid-level moisture will accompany this shortwave trough, and very isolated lightning strikes cannot be ruled out across this region. However, overall coverage is still expected to remain too low to include a general thunderstorm area due to meager instability. A band of showers extends north-south across the western Gulf of Mexico to the central TX Coast this morning in a zone of low-level moisture return and warm air advection. While a lightning strike or two cannot be completely ruled out with this activity along the immediate TX Coast, low-level flow is forecast to slowly weaken across this region by this afternoon/evening. This diminishing low-level flow and warm air advection coupled with no obvious large-scale forcing mechanism for ascent suggest thunderstorm potential should remain low. A weak mid/upper level trough over LA/MS this morning will continue to fill/weaken through the period as it moves slowly eastward across the Southeast. While some low-topped showers have formed across LA in association with this mid/upper-level trough, both lift and instability should remain too limited to support thunderstorms. CLICK TO GET WUUS01 PTSDY1 PRODUCT NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z
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SPC Jan 20, 2018 1730 UTC Day 2 Convective Outlook

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Jan 20, 2018 1730 UTC Day 2 Convective Outlook
Updated: Sat Jan 20 17:00:39 UTC 2018 (Print Version | 20180120 1730Z Day 2 shapefile | 20180120 1730Z Day 2 KML)
Probabilistic to Categorical Outlook Conversion Table
 Forecast Discussion
 SPC AC 201700 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1100 AM CST Sat Jan 20 2018 Valid 211200Z - 221200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF EAST TX INTO SOUTHWEST MO... ...SUMMARY... Isolated severe thunderstorms are possible Sunday afternoon and evening from east Texas into southwest Missouri. A SLGT risk may be warranted for portions of this region if instability is greater than currently expected. ...TX to MO... Latest short-range model guidance continues to suggest a strong southern-stream short-wave trough will eject across the southern Rockies to a position from western KS into west TX by 22/00z as an intensifying 500mb speed max translates into OK. In response to this feature, low-level trajectories will become increasingly favorable for modified moisture to advance inland across TX before advecting into the mid-MS Valley ahead of the short wave late in the period. However, moisture is expected to remain somewhat limited across the warm sector due to the disruptive influence of the short-wave trough that is currently ejecting across the northern Gulf Basin. As a result, a narrow wedge of 50s surface dew points are expected to return as far north as eastern OK/western AR into southwest MO prior to frontal passage Sunday evening, with lower 60s dew points primarily limited to east TX/southern AR/LA. As the trough shifts east it appears strong boundary-layer heating will develop across the southern High Plains into central OK during the afternoon and this should steepen surface-3km lapse rates along a pre-frontal corridor into eastern OK by 21z where values may approach 9 C/km. It appears surface-based convection will develop along a surging dry line over eastern OK/northeast TX between 21-00z and a strongly forced line of thunderstorms should mature along the wind shift by early evening. Forecast soundings across the MRGL risk region suggest near-surface based convection can be expected as far north as southwest MO. Given the strongest large-scale forcing for ascent will be maximized over the mid-MS Valley (240m 500mb height falls during the day) there is reason to believe an organized squall line will advance across eastern OK/northeast TX into AR/MO region after dark. Strong frontal forcing favors linear storm mode and damaging winds are the primary threat, especially as 500mb flow intensifies to near 100kt during the latter half of the period. Even so, a tornado or two can not be ruled out as forecast shear would support organized rotating updrafts. Have opted to maintain 5% severe probs given the relatively benign thermodynamic profiles, but a SLGT risk may be warranted if expected buoyancy improves. ..Darrow.. 01/20/2018 CLICK TO GET WUUS02 PTSDY2 PRODUCT NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0700Z
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SPC Jan 20, 2018 1630 UTC Day 1 Convective Outlook

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Jan 20, 2018 1630 UTC Day 1 Convective Outlook
Updated: Sat Jan 20 16:28:19 UTC 2018 (Print Version | 20180120 1630Z Day 1 shapefile | 20180120 1630Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
 Forecast Discussion
 SPC AC 201628 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1028 AM CST Sat Jan 20 2018 Valid 201630Z - 211200Z ...NO THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorms appear unlikely through tonight across the contiguous United States. ...Synopsis and Discussion... A vigorous southern-stream shortwave trough will move across the Southwest and Great Basin to the Four Corners by early Sunday morning as a Pacific cold front likewise moves eastward across these same areas. Strong forcing for ascent will modestly steepen mid-level lapse rates preceding this shortwave trough across parts of AZ/UT into western CO/NM this afternoon and evening. Some mid-level moisture will accompany this shortwave trough, and very isolated lightning strikes cannot be ruled out across this region. However, overall coverage is still expected to remain too low to include a general thunderstorm area due to meager instability. A band of showers extends north-south across the western Gulf of Mexico to the central TX Coast this morning in a zone of low-level moisture return and warm air advection. While a lightning strike or two cannot be completely ruled out with this activity along the immediate TX Coast, low-level flow is forecast to slowly weaken across this region by this afternoon/evening. This diminishing low-level flow and warm air advection coupled with no obvious large-scale forcing mechanism for ascent suggest thunderstorm potential should remain low. A weak mid/upper level trough over LA/MS this morning will continue to fill/weaken through the period as it moves slowly eastward across the Southeast. While some low-topped showers have formed across LA in association with this mid/upper-level trough, both lift and instability should remain too limited to support thunderstorms. ..Gleason.. 01/20/2018 CLICK TO GET WUUS01 PTSDY1 PRODUCT NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z
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Strong Winter Storm to Impact Portions of Plains and Midwest This Weekend

Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EST Sat Jan 20 2018 Valid 12Z Sat Jan 20 2018 - 12Z Mon Jan 22 2018 ...Precipitation to spread southwestern U.S. this weekend... ...Heavy snows possible for portions of the Great Basin northern and central Rockies Saturday... ...Strong winter storm expected to develop over the central U.S. by Sunday... ...Above normal temperatures persisting across the East... A well-defined cold front and its associated area of precipitation will continue to push southeast into the southwestern U.S. A vast area spanning from the Sierra Nevadas, northern Arizona and northward to the Rockies in Colorado will have a mix of rain and snow today, turning to mostly snow by this evening and through Sunday. The heaviest snow accumulations will likely occur over the mountains of southwest Wyoming, Utah and eastern Nevada this afternoon. And as the front progresses eastward, so will the heaviest precipitation. Locally heavy snow is forecast for the central and southern Rockies. An area of low pressure is expected to strengthen over the southern High Plains tonight and then track northeast into Midwest. In the cold sector, an expansive shield of snow will span from the Rockies into the central High Plains by tonight, reaching the Upper Midwest on Sunday. Snowfall intensity will be moderate to heavy at times and accompanied by gusty winds. A Winter Storm Watch is in effect for eastern Wyoming and Colorado to northern Wisconsin. Further south and ahead of the low's trailing cold front, expect showers and thunderstorms to develop, increasing in coverage and intensity across eastern Oklahoma and Texas into the mid and lower Mississippi valley Sunday afternoon and evening. An active wet pattern will persist this weekend and into early next across the Pacific Northwest. A couple of fronts will pass through the region, spreading widespread rain and mountain snow as it heads toward the Intermountain West. Heavy snow will be possible for portions of the Cascades over the next few days. Temperatures will be near or below normal across the western states. For much of the East coast, expect above normal temperatures to stick around for a few days. Numerous locations will have maximum temperatures remaining 10-20 degrees above their climatological norms. Campbell/Pereira Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.phpgo.usa.gov/3kd2P National Weather Service


SPC Jan 20, 2018 1300 UTC Day 1 Convective Outlook

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Jan 20, 2018 1300 UTC Day 1 Convective Outlook
Updated: Sat Jan 20 12:54:30 UTC 2018 (Print Version | 20180120 1300Z Day 1 shapefile | 20180120 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
 Forecast Discussion
 SPC AC 201254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0654 AM CST Sat Jan 20 2018 Valid 201300Z - 211200Z ...NO THUNDERSTORM AREAS FORECAST... ...SUMMARY... Thunderstorm areas are not expected today and tonight over the contiguous U.S. ...Synopsis... In mid/upper levels, a pronounced, progressive split-flow pattern will cover western North America this period. The southern stream of that split will be dominated by a strong shortwave trough now apparent in moisture-channel imagery from western NV south- southwestward over the CA Channel Islands and well offshore from northern Baja. This trough will move eastward to western portions of CO/NM through the period, with a weak/embedded 500-mb low possibly forming and moving eastward generally across the Four Corners. A somewhat overlapping zone of elevated warm advection, low/middle- level frontal forcing and DCVA-steepened midlevel lapse rates is expected to precede the mid/upper trough this afternoon and tonight -- from the Colorado Plateau of northern AZ/southern UT eastward across the Four Corners to the southern Rockies. Very isolated thunder cannot be ruled out in this regime. However, buoyancy appears too meager to support an area of thunder probabilities at least 10%. Elsewhere, instability and lift should be too weak to support inland thunder associated with a much weaker mid/upper trough crossing the Gulf Coast States and northern Gulf. ..Edwards/Broyles.. 01/20/2018 CLICK TO GET WUUS01 PTSDY1 PRODUCT NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
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