052617 Forecast

Mountain Area Forecast ( May 26-28 )

Severe Thunderstorms Will Impact The Area Saturday Evening & Night – With Severe Thunderstorm Watches And Warnings Likely.  Stay Tuned To NOAA Weather Radio And Favorite Media Sources.

SPC Mesoscale Discussion

The Storm Prediction Center Has Issued A Slight Risk Of Severe Thunderstorm Development For Saturday Into Sunday ( May 27-28 )

Storm Prediction Center Updated Risk Regions

A Potential Derecho, Impacting Areas From Missouri Across Kentucky and Tennessee, Will Become A Major Weather Force Into Saturday Night And Early Sunday As It Moves Toward The Appalachians

Potential Derecho ( Long-lived Wind Damage ) Developing Over Missouri At 4:18 PM

**A Risk Of Flooding Rainfall Will Exist Throughout The Memorial Day Holiday Weekend

Overnight Into Friday Morning

Rain showers and areas of drizzle, ending overnight into morning.  Low clouds.  Areas of dense fog ( widespread at upper elevations then cloud bases lifting into morning ).  Winds WSW-W at 5-15 mph, with higher gusts, on ridges and plateaus below 2700 feet.  Winds W-WNW 10-20 mph, with higher gusts, along mountain ridges above 2700 feet.  Temperatures widespread in the 50s, except near steady or slowing rising to around 50 degrees on highest ridges.

Friday Afternoon

Partly cloudy with increasing high clouds by late.  Warmer.  Winds W-SW 5 to 15 mph with higher gusts.  Temperatures varying from 60s in upper elevations to the mid-upper 70s.

Friday Night Into Saturday Morning

Partly to mostly cloudy ( high clouds ).  Areas of dense valley fog.  Winds SSW-WSW at 5-15 mph, with higher gusts, on middle-upper elevation mountain ridges. Temperatures varying from mid-upper 40s to lower 50s within cooler mountain valleys to the lower-middle 60s.

Saturday Afternoon

A chance of showers & downpours in thunderstorms.  Some storms could be strong to severe.  Humid.  Winds SW-WSW at 5-15 mph, with higher gusts.  Temperatures varying from upper 60s to lower 70s at highest elevations to upper 70s to lower 80s.

Saturday Night Into Sunday Morning

A chance of showers & downpours in thunderstorms.  Some storms could be strong to severe.  Humid & hazy.  Areas of fog possible.  Winds SW at 5 to 15 mph with higher gusts.  Temperatures widespread in the 60s.

Sunday Morning Into The Afternoon

A chance of showers & downpours in thunderstorms.  Some storms could be strong to severe.  Winds SW-W at 5-15 mph with higher gusts.  Temperatures varying from 60s within upper elevations to the lower-middle 70s.


Weather Discussion ( Stormy )

Early Overnight of Saturday Update

A beautiful ending to Friday was a nice way to kick off the Memorial Holiday Weekend.  Unfortunately, these pristine conditions will not last with increasingly humid & unstable air poised to build across the mountain landscape ( MCS ).

Beautiful Friday PM From Powell Valley Overlook – May 26, 2017

The Storm Prediction Center continues to be very aggressive with an outbreak of severe thunderstorms Saturday.  While some strong to severe thunderstorms may develop locally over the mountains as afternoon instability increases, the main concern by late Saturday & Saturday Night could be  formation of a Mesoscale Convective System ( MCS ).

Storm Prediction Center Wind Damage Risk From 8 AM Saturday to 8 AM Sunday

The MCS develops initially over the Mississippi Valley then rolls eastward across Kentucky and Tennessee toward the Appalachians with a wind damage threat ( there is some potential for a long-lived wind damage producing MCS to form given the instability gradient that will be developing across the Mississippi and upper Tennessee valleys ).

That is the preliminary scenario.  Stay tuned for updates as the atmosphere responds to actual thunderstorm formation during Saturday ( with luck any such system will weaken by the time it reaches the mountains, but that can not yet be counted on so that everyone should remain alert and have ways to receive watches-warnings that will be issued ).


Previous Discussion

A stormy May 2017 weather pattern will continue through the Memorial Day Holiday period, with very unstable air developing over the mountain region as another frontal boundary stalls across the Mountain Empire.

*Temperatures in the 40s, with wind chills in the 30s, were observed throughout the daylight hours of Thursday at upper elevations in the High Knob Massif which were engulfed in orographic clouds ( dense fog ) and persistent rain showers.
Widespread 50s were observed at lower-middle elevations under an upper level low.  These conditions will change radically into this Memorial Day Holiday Weekend.
European Model 850 MB Analysis At 8:00 AM Thursday – May 25, 2017

Given a saturated landscape this is not good news, with an enhanced risk for flooding rains due to a combination of antecedent conditions ( wet April-May ) and downpours within showers and thunderstorms.

Antecedent Conditions

Clintwood 1 W NWS

April 1 to May 25 Rainfall
6.76 + 7.61 = 14.37″

*April 1 to May 1-25 rainfall totals as great as 16.00″ to 18.00″ have been observed across the southern Appalachians, from western to eastern slope sites ( e.g., High Knob Massif, Grayson Highlands State Park, Grandfather Mountain ).
**Flooding rains in the Town of Clintwood during May 24 were caused by days of significant rainfall prior to the 0.63″ that officially fell into the rain gauge = the importance of antecedent conditions which allowed me to be able to forecast flooding rains before they developed.  The potential for more flooding will have to be respected through the Memorial Day Holiday Weekend.
NAM 12 KM Model Total Rainfall Forecast To 8:00 AM Monday – May 29, 2017

While forecast models will struggle to pinpoint locations of heaviest rainfall, again the signal and atmospheric setting is clearly indicating showers & flooding downpours within thunderstorms, some of which may be strong-severe.

NAM Model Precipitable Water Forecast At 8:00 PM Saturday – May 27, 2017

In addition to another high precipitable water air mass along an west-east stalling boundary, this returning air mass is also forecast to become very unstable.

NAM Model CAPE Forecast At 8:00 PM Saturday – May 27, 2017

An axis of very high Convective Available Potential Energy, called CAPE, will extend across much of Kentucky toward the western side of the Appalachians by later Saturday.

NAM Model Supercell Composite Forecast At 5:00 PM Saturday – May 27, 2017

The Supercell Composite forecast, formed in part by the amount of cross-over which occurs vertically between the 850 MB and 500 MB levels, is also predicted to reach rather significant values.

Storm Prediction Center Severe Probabilities
Storm Prediction Center Discussion For 8 AM Saturday To 8 AM Sunday

These risk regions may be altered as the time for this event gets closer, as well as extended through the remainder of this Holiday Weekend ( check back for updates ).

NAM Model 925 MB Theta-E Forecast At 8:00 AM Sunday – May 28, 2017

A plume of rich moisture, with elevated values of 925 MB theta-e ( equivalent potential temperature ) possesses a subtropical connection from the Gulf of Mexico and will continue the potential for flooding rains throughout the Memorial Day Holiday period.

NAM Model 250 MB Jet Stream Forecast At 8:00 AM Monday – May 29, 2017

A developing jet streak in upper level wind fields by late in the weekend and Memorial Day will need to be followed for enhancement of showers-thunderstorms over the southern Appalachians and Tennessee Valley.

These parameters, and many more, will need to be closely followed by forecaster’s heading into this weekend.  Stay tuned to NOAA Weather Radio and your favorite media sources for watches and/or warnings which will likely be needed.