Area Forecast Discussion
Issued by NWS Blacksburg, VA
Issued by NWS Blacksburg, VA
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015 FXUS61 KRNK 050247 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1047 PM EDT Thu Jul 4 2024 .SYNOPSIS... Exceptionally warm and muggy air continues to pool to the south of a stationary front situated near the Ohio River, while a series of upper level disturbances passing across the region will trigger rounds of spotty showers and thunderstorms through Saturday. A weak cold front will pass across the Mid-Atlantic on Saturday night into early Sunday. Spotty showers and thunderstorms will remain in the forecast for early next week as an upper level trough develops over the central United States. && .NEAR TERM /THROUGH FRIDAY/... As of 1045 PM EDT Thursday... Key Messages: - Probability of rain lowered for the rest of the night. - Added fog for tonight Showers have ended across the region. No change to the chance of rain in southeast West Virginia Friday morning. IR satellite images showers high clouds rapidly eroding over the mountains and foothills. Fog was starting to develop as the clouds cleared out. Made minor adjustments to temperatures for the rest of the night based on current observations and trends shown in the NBM and gridded LAV guidance. Previous discussions: As of 810 PM EDT Thursday... Leading edge of band of thunderstorms was advancing into central North Carolina and eastern Virginia. Showers behind the more intense part of this band extended back into the Virginia and North Carolina piedmont. Based on the current speed and direction, the west edge of the rainfall will be east of Charlotte and Buckingham Counties by 11PM. As clouds clear out late this evening, expecting fog to develop. No changes to overnight temperatures at this time. Surface dew points in the 70s will result in muggy conditions for the rest of the evening. As of 215 PM EDT Thursday... Keeping an eye on scattered showers and a few thunderstorms which have developed across the mountains this afternoon in an exceptionally warm and humid airmass. Precipitable water measured from this morning`s sounding was 1.72 inches, and it`s likely that value has now risen above 1.8 inches as deep moisture continues to pool ahead of a stationary front draped near the Ohio River. Surface observations in spots where these cells have passed overhead indicate they are efficient rainfall producers, with rates in a few of them reaching 1" to 2" per hour. However, the activity is expected to remain disorganized and progressive, so believe the flooding threat will remain localized. The severe threat will remain localized as well given downsloping with the westerly motion of this activity, in addition to limited shear aloft, both of which help to limit storm intensification. The other concern is heat index values reaching above 100 in spots across the Piedmont through around 5 pm this evening north of Highway 460, when showers and storms will bring rain-cooled relief to the area. Locations further south, especially the Southside area, may miss out on the scattered activity until early evening when storms arrive in that area. Rapid update weather forecast models seem to be in good agreement that showers and storms will have diminished by 8 pm for areas west of Highway 29, with activity fading quickly further east. Expect warm and muggy conditions overnight, with overnight lows ranging from the mid and upper 60s across the mountains, to the low 70s across the Piedmont. For Friday, upper level ridging over the Mid-Atlantic will remain in place, with winds shifting more southwesterly. With no change in airmass across our area, will again experience unusually warm conditions across the region. Afternoon heat index values are expected to reach above 105 degrees for large portions of the Piedmont, most likely during the 1 pm to 5 pm timeframe, but will issue a Heat Advisory from 11 am through 8 pm for those few areas that heat up sooner and/or remain hot longer. Subtle upper level disturbances will again pass across the region as well, and will trigger another round of spotty showers and thunderstorms. This activity will again be capable of producing locally heavy downpours and gusty winds. Confidence in the above scenario is moderate to high. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 1215 PM EDT Thursday... Key Points: 1. Cooler weather than what is expected today and Friday but still slightly above normal. 2. Good chances of heavy rain producing showers and storms along and advance of a cold front passage Friday night through Saturday. A look at the 4 July 00Z NCEP Ensemble Mean 500 mb Geopotential heights shows a shortwave trough moving from the Upper Mississippi River Valley into the Great Lakes Friday night into Saturday. Saturday night through Sunday night a longer wave trough develops over the Upper and Middle Mississippi River Valleys, and remains nearly stationary through this period. At the surface, Friday night into and through Saturday, an area of low pressure will progress from Michigan to southern Quebec. Its associated cold front will first approach, and then cross our region. Saturday night into Sunday, high pressure moves over our area, but the cold front will still need to exit our area on Saturday. By Sunday night, the surface high weakens in response to an approaching front over the Upper Mississippi Valley and winds across our region trend south. Output from the 4 July 00Z Ensemble Situational Awareness Table shows 850mb temperatures Friday night into Saturday within the +18C to +21C range across the area with the high end of this range across eastern sections of the region. Additionally, areas with values of +20C to +21C fall within the 90 to 97.5 percentile of the 30-year climatology. For Saturday night through Sunday night, values drop slightly and are expected to be more within the +18C to +20C range. A look at Precipitable Water (PW) values shows a distinct maximum around 2.00 inches immediately in advance of an west to east moving cold front Friday night. The farther away from this front, especially to its west, the lower the PW. Values over roughly 1.75 inches fall within the 99+ percentile of the 30-year climatology. On Saturday, with the cold front still not quite out of the the region, still expected PW value around 1.75 to 2.00 inches across the far southeast portion of the region. Far western parts of the region will be closer to 1.25 inches. Saturday night through Sunday night, PW values of 1.25 to 1.75 will be common across the area, with the highest amounts across southeast sections. The above weather scenario offers a Friday night with organized convection in the form of showers and storms moving west to east across the region. The greatest concentration should be immediately along and advance of cold front. This front does not clear the southeast portion of the area until either the late afternoon or early evening on Saturday. Given the very high PW values for this time of year, locally heavy rain could be possible from any of the stronger showers or storms. Effective bulk shear is expected to be weak, or around 20 to 25 kts nearly parallel to the orientation of the cold front. This pattern could more easily allow for training of showers/storms, adding to the potential for locally heavy rain accumulations. Given our current drought conditions, any flooding from this activity would be likely confined to locations of poor drainage or urban effects. On Sunday, high pressure is expected to take a firmer hold on our weather pattern allowing for drier and less humid conditions. The one question mark will be the southeast portion of the region. This area may still be close enough to the departing front to have some isolated to scattered showers/storms during the afternoon hours. Sunday night, if any area was not already rain-free during the daytime, it will become rain-free during the night. Confidence during this portion of the forecast is moderate to high with the biggest question mark being the position of the cold front Saturday night into Sunday and its influence on our weather. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 1215 PM EDT Thursday... Key Points: 1. Above normal temperatures. 2. Active period with daily chance of showers and storms. A look at the 4 July 00Z NCEP Ensemble Mean 500 mb Geopotential heights shows a long wave upper level trough over the Middle and Upper Mississippi River Valleys that slowly flattens and shifts northeast into and across the Great Lakes region by Thursday. Concurrent to this slow progress of the upper trough, high pressure builds over the Southwest US with the western extent of another upper ridge over the Southeast US. At the surface, high pressure will remain across the western Atlantic. A slow moving trough will inch through the Great Lakes and Ohio Valley region, keeping our area with a good warmth and moisture feed. While this trough is the result of the ensemble averaging, a look at some deterministic guidance suggests perhaps a cold front passage that either stalls over the region, or returns north as a warm front. Output from the 4 July 00z Ensemble Situational Awareness Table shows 850mb temperatures are expected to range from +18C to +20C on Monday and Tuesday, and be slightly cooler in the +17C to +19C range Wednesday and Thursday. The values on Monday and Tuesday at the top end of the range touch the 90 to 97.5 percentile of the 30-year climatology. Regard the PW, values creep back higher Monday through Thursday with values within a couple tenths of an inch of 1.75 inches across the region. The above weather scenario will place our region within a continuous pattern of good feed of warmth and moisture into the region. This will promote the lower portions of the troposphere to be generally more unstable with afternoon heating and/or with the passage of some form of lifting mechanism. While the broad ensemble offers the region will remain on the southeast side of persistent central CONUS trough, individual ensemble members and other deterministic guidance offers the potential of a cold front passage sometime either Tuesday or Tuesday night. This front then either stalls over the region, or after shifting south, returns north again over the region as a warm front. Regardless, our overall pattern will be one with daily chances of showers/storms with above normal temperatures. Given the uncertainties in the position/timing of any fronts impact our region, confidence is not high in trying to pinpoint when any particular region or timeframe will be likely to see measurable precipitation. Am still inclined to keep POPs sub-60 percent. However, if I had to pick one day of the four to place likely POPs, Tuesday/Tuesday night across western sections, would be my choice. Will need to explore the thoughts of neighboring offices and collaborate accordingly. Confidence in this portion of the forecast is moderate to high regarding temperatures and the fact that it will be active with showers and storms. However, confidence is not very high in terms of timing and location of time periods with the highest probability save what what discussed regarding Tuesday/Tuesday night. && .AVIATION /03Z FRIDAY THROUGH TUESDAY/... As of 820 PM EDT Thursday... Leading edge of band of thunderstorms was advancing into central North Carolina and eastern Virginia. Showers behind the more intense part of this band extended back into the Virginia and North Carolina piedmont. Based on the current speed and direction, the west edge of the rainfall will be east of KDAN by 02Z/10PM. As clouds clear out late this evening, expecting MVFR to IFR fog to develop. KBCB and KLWB may have a few hours of LIFR fog after midnight. Little change in the airmass is expected for Friday. Fog will burn off quickly by late morning. Scattered thunderstorms will redevelop between 16Z-19Z with daytime heating, forming mainly along the Blue Ridge before spreading in coverage as the afternoon progresses. MVFR visibility due to very heavy rain should be expected with these storms. Average confidence for ceiling, precipitation, wind. Below average confidence for visibility. OUTLOOK Friday night through Tuesday... Friday night through Tuesday: SCTD SHRA/TSRA remain possible as each afternoon and evening. Flight category reductions are likely, especially Saturday and Tuesday afternoon and evening. Sunday and Monday the probability of showers and thunderstorms decreases, but remains possible in the afternoons and evenings. Prevailing conditions will be VFR. && .CLIMATE... As of 300 PM EDT Thursday... Record highs and warmest lows temperatures for July 4th and 5th. Thursday 07/04/2024 Site MaxT Year HiMin Year KBLF 92 1957 68 2018 KDAN 99 1925 74 1999 KLYH 98 1966 76 1902 KROA 99 1999 74 2018 KRNK 94 1966 71 1941 Friday 07/05/2024 Site MaxT Year HiMin Year KBLF 93 1948 71 1969 KDAN 103 1919 76 1926 KLYH 98 2012 75 1900 KROA 100 1999 76 2012 KRNK 94 1930 78 1949 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Heat Advisory from 11 AM to 8 PM EDT Friday for VAZ032>034- 043>047-058-059. NC...Heat Advisory from 11 AM to 8 PM EDT Friday for NCZ003>006-020. WV...None. && $$ SYNOPSIS...NF NEAR TERM...AMS/NF SHORT TERM...DS LONG TERM...DS AVIATION...AMS/NF CLIMATE...RCS/SH