Brett Adair
08-25-2004, 11:12 PM
http://www.editor.usaweather.net/severe_view.php?day=1
[SWDDY1]
USA Weather - Severe Weather Division
Day 1 Severe Weather Outlook
12:00 AM EDT Thursday, August 26, 2004
Severe Weather Forecast For Thursday, August 26, 2004
Valid 0500z Thursday - 0500z Friday
---Significant Severe Weather Event Possible Across Portions of The Western Great Lakes---
National Synopsis
Broad, low amplitude trough will exist across the Western and Central Conus with a powerful jet stream snaking around its periphery. A massive Bermuda High dominating east coast weather will cause strong southerly flow to develop on its western edge across the Mississippi River Valley. This strong southerly flow will act to draw a very tropical airmass northward to the Great Lakes region. This tropical airmass will interact with a sweeping cold front to produce showers and thunderstorms across much of the Central United States. Cold air aloft will create some instability, and thus isolated showers and thunderstorms across much of the Northwestern US and the Northern Rockies.
Upper Mississippi River Valley
Attention: DLH, MNP, DVN, GRB, CHI
Only two potential inhibitors of explosive thunderstorm development across the region Thursday Afternoon and Evening...
(1) Cloud cover/stratiform precipitation in the morning hours across N MN, N WI and the UP of MI. Any thick overcast or even BKN cloud decks particularly in the low-mid levels would significantly decrease chances of severe weather. However it appears any such cloud cover would be associated with isentropic upglide invof Lake Superior so the entire area would not be impacted. Even so, earlier coordination and latest ETA model run (00 utc) suggests all cloud cover will lift into Canada during the morning, leaving much of the region sunny...making for effective destabilization of the boundary layer. As a result, have not let any such concerns affect categorical risks across the concerned area.
(2) A strong capping inversion that would most likely be present from Southern Iowa and Southeastern Nebraska southward into Northern Missouri and Northern Kansas. This cap would be resultant of strong waa in the 800:700mb layer, significantly shallowing out lapse rates and inhibiting any convective development. As a result of this potential negative factor, have decided to only place this area under Very Slight Risk to Slight Risk (categorical).
However, across the rest of the potentially significant event area, all parameters are phasing together for a very active severe weather day. On the side of thermodynamics, low-level mositure will be aplenty and plenty of insolation should help bring low-level lapse rates to dry adiabatic by early afternoon...bringing temperatures up into the 80s/90s F. The combination of the very warm surface temperatures and high dewpoint air will produce widespread surface-based CAPE values of 3000-5000 j/kg. Mid-level lapse rates are also fairly steep...suggesting updraft strength will be fairly strong and supportive of organized severe weather. Kinematic environment is also very conducive to a severe weather event. Sly lower tropospheric jet and band of 105+ knot westerly upper tropospheric winds will cross to produce richly curved hodographs...primarily across Central and Southern Wisconsin through Southeast Minnesota and southward. Kissing jets should allow for rather impressive vertical motion fields.
Thunderstorms are expected to initiate along the cold front between 18Z and 20Z in far E MN and NW WI and then gradually build swwd to (and eventually past) the Iowa border. Initial convective mode should be supercellular given the veering, deep shear profiles...but evolution to one or more upscale MCSs is likely with time as the tstms push ewd towards Lake Michigan. Additionally, a stronger band of low level winds may act to enhance vertical motion in WC/C IA on the edge of the capping inversion to locally produce thunderstorms. These thunderstorms would exhibit strong supercellular characteristics and the area would likely require an upgrade to high risk if this initiation becomes apparent.
Large hail to 3 inches in diameter and damaging winds to 80 mph appear to be the primary threats although tornadoes cannot be ruled out. A couple of strong tornadoes will be possible given the degree of buoyancy...updrafts will easily be able to pull any HCR's vertical and produce copius storm rotation.
Southeast Wyoming, Northeast Colorado and Panhandle of Nebraska
A pocket of marginal instability will develop by late afternoon across the area with LI's getting down to -2 to -3C. A series of impulses will ride a 125 knot jet streak across the area and do some "layer-lifting" to help generate showers and thunderstorms. A low-level circulation will also aid in enhancing low level convergence. It appears that if thunderstorms do develop in the confined pocket of instability they would have a highly sheared environment to interact with and a few may become severe with large hail and damaging winds the primary threats.
Eastern North Dakota into Northern Minnesota
Ongoing thunderstorms at the time of the outlook may continue with an isolated severe weather threat into North-Central Minnesota from Eastern North Dakota. A compact but potent bow echo continues to race enewd across the area and should have some marginal instability to work with over the next several hours. PBL moistening may eventually put an end to severe wind gusts but marginally severe hail cannot be ruled out throughout the overnight. This bow echo will be monitored through 05Z and if it still poses a severe weather threat an isolated pocket of categorical Slight Risk may be required.
LAMERS
[SWDDY1]
USA Weather - Severe Weather Division
Day 1 Severe Weather Outlook
12:00 AM EDT Thursday, August 26, 2004
Severe Weather Forecast For Thursday, August 26, 2004
Valid 0500z Thursday - 0500z Friday
---Significant Severe Weather Event Possible Across Portions of The Western Great Lakes---
National Synopsis
Broad, low amplitude trough will exist across the Western and Central Conus with a powerful jet stream snaking around its periphery. A massive Bermuda High dominating east coast weather will cause strong southerly flow to develop on its western edge across the Mississippi River Valley. This strong southerly flow will act to draw a very tropical airmass northward to the Great Lakes region. This tropical airmass will interact with a sweeping cold front to produce showers and thunderstorms across much of the Central United States. Cold air aloft will create some instability, and thus isolated showers and thunderstorms across much of the Northwestern US and the Northern Rockies.
Upper Mississippi River Valley
Attention: DLH, MNP, DVN, GRB, CHI
Only two potential inhibitors of explosive thunderstorm development across the region Thursday Afternoon and Evening...
(1) Cloud cover/stratiform precipitation in the morning hours across N MN, N WI and the UP of MI. Any thick overcast or even BKN cloud decks particularly in the low-mid levels would significantly decrease chances of severe weather. However it appears any such cloud cover would be associated with isentropic upglide invof Lake Superior so the entire area would not be impacted. Even so, earlier coordination and latest ETA model run (00 utc) suggests all cloud cover will lift into Canada during the morning, leaving much of the region sunny...making for effective destabilization of the boundary layer. As a result, have not let any such concerns affect categorical risks across the concerned area.
(2) A strong capping inversion that would most likely be present from Southern Iowa and Southeastern Nebraska southward into Northern Missouri and Northern Kansas. This cap would be resultant of strong waa in the 800:700mb layer, significantly shallowing out lapse rates and inhibiting any convective development. As a result of this potential negative factor, have decided to only place this area under Very Slight Risk to Slight Risk (categorical).
However, across the rest of the potentially significant event area, all parameters are phasing together for a very active severe weather day. On the side of thermodynamics, low-level mositure will be aplenty and plenty of insolation should help bring low-level lapse rates to dry adiabatic by early afternoon...bringing temperatures up into the 80s/90s F. The combination of the very warm surface temperatures and high dewpoint air will produce widespread surface-based CAPE values of 3000-5000 j/kg. Mid-level lapse rates are also fairly steep...suggesting updraft strength will be fairly strong and supportive of organized severe weather. Kinematic environment is also very conducive to a severe weather event. Sly lower tropospheric jet and band of 105+ knot westerly upper tropospheric winds will cross to produce richly curved hodographs...primarily across Central and Southern Wisconsin through Southeast Minnesota and southward. Kissing jets should allow for rather impressive vertical motion fields.
Thunderstorms are expected to initiate along the cold front between 18Z and 20Z in far E MN and NW WI and then gradually build swwd to (and eventually past) the Iowa border. Initial convective mode should be supercellular given the veering, deep shear profiles...but evolution to one or more upscale MCSs is likely with time as the tstms push ewd towards Lake Michigan. Additionally, a stronger band of low level winds may act to enhance vertical motion in WC/C IA on the edge of the capping inversion to locally produce thunderstorms. These thunderstorms would exhibit strong supercellular characteristics and the area would likely require an upgrade to high risk if this initiation becomes apparent.
Large hail to 3 inches in diameter and damaging winds to 80 mph appear to be the primary threats although tornadoes cannot be ruled out. A couple of strong tornadoes will be possible given the degree of buoyancy...updrafts will easily be able to pull any HCR's vertical and produce copius storm rotation.
Southeast Wyoming, Northeast Colorado and Panhandle of Nebraska
A pocket of marginal instability will develop by late afternoon across the area with LI's getting down to -2 to -3C. A series of impulses will ride a 125 knot jet streak across the area and do some "layer-lifting" to help generate showers and thunderstorms. A low-level circulation will also aid in enhancing low level convergence. It appears that if thunderstorms do develop in the confined pocket of instability they would have a highly sheared environment to interact with and a few may become severe with large hail and damaging winds the primary threats.
Eastern North Dakota into Northern Minnesota
Ongoing thunderstorms at the time of the outlook may continue with an isolated severe weather threat into North-Central Minnesota from Eastern North Dakota. A compact but potent bow echo continues to race enewd across the area and should have some marginal instability to work with over the next several hours. PBL moistening may eventually put an end to severe wind gusts but marginally severe hail cannot be ruled out throughout the overnight. This bow echo will be monitored through 05Z and if it still poses a severe weather threat an isolated pocket of categorical Slight Risk may be required.
LAMERS