Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 14 2023 08:13:04 FOUS30 KWBC 140813 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 412 AM EDT Mon Aug 14 2023 Day 1 Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID WEST AND LOWER MICHIGAN, FROM THE SOUTHERN TO CENTRAL APPALACHIANS, MID ATLANTIC AND SOUTHEAST NEW YORK STATE INCLUDING THE NEW YORK CITY METRO AREA AND LONG ISLAND... ....Mid West into Lower Michigan... A strong, compact upper low will be pressing eastward Monday through portions of the Mid-West and south of the Lower Lakes.=20 Simulated radars from the latest hi res runs all show a well defined comma head deformation band moving across southern Wisconsin, northern Illinois into the southern portions of the L.P. of Michigan. While instability is not expected to be very high, Mu-cape values generally less than 500 j/kg, the slow motion of the upper low in a region of 1.50-1.75" PW values will provide for a 4 to 6 hour period of moderate to heavy rainfall with hourly rates of .50-1" possible. Northeasterly low level flow off of southern portions of Lake Michigan may also maximize boundary layer convergence along the southeast Wisconsin coast into the northeast Illinois coast. HREF neighborhood probabilities are showing this area having the greatest probabilities, 80%+ for 3"+ totals day 1 and a smaller region of 40-60% probabilities for 5"+ totals. Overall, the slight risk area was drawn to encompass where the greatest 2 and 3"+ HREF probabilities are for the upcoming day 1 period, along where the HREF EAS is showing the best agreement for 1 and 2"+ amounts. The latest outlook was narrowed from the previous issuance as per the above mentioned HREF neighborhood and EAS probabilities. ....Southern to Central Appalachians, Mid-Atlantic into southeast New York State... Anomalous PW values, 2 to 2.5+ standard deviations above the mean, expected to surge northeastward day 1 on the southeast side of the strong upper low moving from the Mid-West and just south of the Lower Lakes. Strong uvvs ahead of this closed low will support heavy rain potential in this anomalous PW axis that will push northeastward day 1 from the Southern to Central Appalachians, Mid-Atlantic and into southeast New York State.=20 There is general model consensus for an axis of heavy rains across these areas, with the slight risk area maintained primarily where the highest HREF neighborhood probabilities for 2"+ are depicted along with where the greatest HREF EAS probabilities for 1"+ are depicted. Changes to the previous outlook were to expand the slight risk area northward by approximately 40 to 50 miles along the northeast PA and Southern Tier of NY border area.=20=20 Instability is expected to be limited across this area, but with lower ffg values from recent rains, runoff issues are possible from .50-1"+ totals in an hour or two in the region of max isentropic lift ahead of the surface wave pushing across the northern Mid-Atlantic into southeast NY State. ....Southwest into the Southern Great Basin and northern Sierra... A surface cold front currently sinking southward across the Southern High Plains is expected to become stationary in the vicinity of the AZ/NM border. The simulated hi res radars suggest this boundary will be the focus for convection Monday afternoon in an axis of 500-1000 j/kg along this front and PW values 1 to 2+ standard deviations above the mean. HREF neighborhood probabilities are fairly high in this axis for 1"+ amounts, although the HREF EAS for 1"+ are not as high. Isolated runoff issues are possible, but with low EAS values 5-15% the risk level was maintained at marginal. The previous marginal risk area was expanded westward into the Southern Great Basin and into the Northern Sierra. This was to cover what may be another day of widely scattered convection in a broadly difluent upper flow to the east of the slow moving upper low off the south central California coast. Oravec Day 2 Valid 12Z Tue Aug 15 2023 - 12Z Wed Aug 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST VIRGINIA INTO PORTIONS OF EAST CENTRAL NORTH CAROLINA... ....Southeast Virginia into east central North Carolina... The trailing frontal boundary from the surface forecast to move from the Northern Mid-Atlantic into New England day 2 will be pressing slowly southeastward through the coastal Mid-Atlantic into the Southeast. PW values 2.5 to 2+ standard deviations above the mean expected to persist along and ahead of this boundary day 2. Very favorable right entrance region jet dynamics and defined boundary layer convergence along and ahead of the cold front in the high PW axis will support potential for heavy rainfall totals along the boundary. Model consensus for the greatest amounts are from the Hampton Roads area of southeast Virginia, southwestward along and ahead of this front into east central portions of North Carolina. The previous slight risk area was narrowed to better match the model consensus for heavy totals along and ahead of this boundary. The marginal risk extending southwestward through the Southeast into the eastern Gulf Coast was also narrowed to better match the heavy rain potential along the front, removing it from Northwest Florida. No changes to the risk level here as ffg values remain high. ....Southwest into the Southern Great Basin... The surface frontal boundary acting as a focus for convection day 1 across the AZ/NM border region is expected to weaken by day 2.=20 However, broadly difluent upper flow to the east of the upper trof moving slowly northward off the central California coast will again support potential for widespread scattered convection from the Southwest into the Southern Great Basin and Northern Sierra.=20 With PW values expected to remain above average across these areas, 1 to 2+ standard deviations above the mean, isolated heavy rains and localized runoff issues are possible, although confidence is very low in where it may occur, supporting the maintenance of a broad marginal risk area. Oravec Day 3 Valid 12Z Tue Aug 15 2023 - 12Z Wed Aug 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE SOUTHEAST COAST, NORTH FLORIDA AND THE URBAN AREAS OF SOUTHWEST AND SOUTHEAST FLORIDA... ....Carolina, Georgia Coasts into Florida... The surface frontal boundary pressing slowly eastward along the east coast day 2, will become stationary day 3 as the upper trof weakens. PW values will remain above average along the immediate NC,SC and GA coasts and across FL, with values 2+ standard deviations above the mean. While the upper trof is weakening day 3, uvvs may still be enhanced by right entrance region jet dynamics that linger along the immediate coast. Model consensus is for heavy rainfall potential in the vicinity of this front and south across much of Florida. The only change to the previous marginal risk area was to remove it across south central FL away from the urban areas of Southwest and Southeast FL. ....Southwest into the Southern Great Basin... Similar to the day 1 and day 2 period, a broad marginal risk area depicted across the Southwest into the Southern Great Basin. The upper low that was moving slowly northward day 2 off the central California coast is expected to become stationary day 3. This will help to maintain a broadly upper flow pattern to its east.=20 PW values will remain above average across the marginal risk area, supporting isolated heavy rainfall totals from what may again be an afternoon of widely scattered convection.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8zogV7JKa3fCQjj94U2wBfk7nLLsm6-axbAX52R5vmiY= 1InDuuPss5_5tg7_233v5PL5_2HsNQv5f2oOzFY7bYgdFFQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8zogV7JKa3fCQjj94U2wBfk7nLLsm6-axbAX52R5vmiY= 1InDuuPss5_5tg7_233v5PL5_2HsNQv5f2oOzFY7yuXNII8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8zogV7JKa3fCQjj94U2wBfk7nLLsm6-axbAX52R5vmiY= 1InDuuPss5_5tg7_233v5PL5_2HsNQv5f2oOzFY7_RKWsAo$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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