textproduct: NWS Wilmington

WHAT HAS CHANGED

Thunderstorm chances have increased for this afternoon, mainly across the Cape Fear region.

KEY MESSAGES

1) Deep moisture and the approach of a weak disturbance aloft should trigger waves of showers and thunderstorms this afternoon.

2) Hot temperatures and seasonably high dewpoints may push heat indices over 105 degrees Tuesday through Friday.

3) Thunderstorm chances should increase again Friday and Saturday with the approach of a cold front from the north.

DISCUSSION

KEY MESSAGE 1...Deep moisture and the approach of a weak disturbance aloft should trigger waves of showers and thunderstorms this afternoon.

Bermuda high pressure centered just south of Bermuda is directing a flow of moisture from the Gulf and Bahamas northward across the Carolinas. Precipitable water values of 2.0 to 2.2 inches will exist today. A weak shortwave should arrive from the northwest at the time of peak heating this afternoon, coincident with surface-based CAPE exceeding 3000 J/kg. Hourly HREF fields suggest convection will develop in multiple clusters this afternoon, moving east-southeastward along the surface-to-6 km mean wind vector. Storm coverage is expected to be largest in the Cape Fear region where forecast PoPs are as high as 60 percent.

While no organized severe weather is anticipated, gusty winds and locally heavy rainfall could occur in line segments sculpted by 0-6 km bulk shear around 20 kt. Storms could also become prolific lightning producers given a significant amount of CAPE above the -30C level and with parcel equilibrium levels at the base of the stratosphere.

Some showers and thunderstorms could linger into the early evening as the shortwave reaches the coast.

KEY MESSAGE 2...Hot temperatures and seasonably high dewpoints may push heat indices over 105 degrees Tuesday through Friday.

The Bermuda high will shift south Tuesday, then expand a ridge axis westward across the Gulf Coast Wednesday and Thursday. The westerly flow this will create across the Carolinas will bring a hot airmass overhead with 850 mb temps expected to exceed +20C through the period. Mixing this down to the surface implies high temperatures away from the coastline should reach the mid 90s, perhaps even upper 90s in spots. With dewpoints remaining seasonably high in the 70s, forecast heat indices should reach 105-110 degrees for most areas beginning Tuesday and heat advisories are likely.

It's worth noting local temperature verification numbers show the GFS has been the worst performer for high temperatures over the past 2, 5, and 10 days. GFS MOS output is notably warm compared to other guidance this week.

KEY MESSAGE 3...Thunderstorm chances should increase again Friday and Saturday with the approach of a cold front from the north.

The upper air pattern should amplify late this week as a longwave trough takes shape just off the East Coast and the former Bermuda ridge retrogrades to the Mississippi Valley. Northwest flow aloft may push a cold front southward into the Carolinas for the weekend, accompanied by cooler temperatures and an uptick in the coverage of showers and thunderstorms.

AVIATION /06Z MONDAY THROUGH FRIDAY/

Winds just off the surface are a little stronger tonight versus the past few nights, but moisture profiles are very similar. With more mixing near ground level we're not likely to see visibility restrictions from fog, however there's still a moderate potential we'll see IFR cloud ceilings appear at KFLO and KLBT before sunrise. Dewpoint depressions are a couple degrees larger at 06z compared to this same time last night, so I've delayed the onset of these low clouds by a couple hours.

VFR conditions should develop across the board around 13z. Warming temperatures this afternoon and the approach of a weak upper level disturbance should trigger the development of showers and thunderstorms mainly after 18z. The potential for direct impacts appears highest at KILM where a TEMPO group has been placed in the TAF 18-22z. Other airports have a moderate potential for impacts and PROB30 was used in the forecast at KCRE, KMYR, KFLO, and KLBT.

Extended Forecast... VFR should prevail through the rest of the week. Shallow ground fog is possible inland each early morning. The approach of a cold front from the north may lead to increasing coverage of showers and thunderstorms again by Friday.

MARINE

Bermuda high pressure centered just to the south of Bermuda will become suppressed farther south over the next few days as a series of low pressure systems move across southeastern Canada and New England, lowering surface pressures across the northwest Atlantic. A very weak piece of one of these disturbances will move southeastward across the coastal Carolinas this afternoon into this evening, likely triggering a wave of showers and thunderstorms. Gusty winds are possible with these storms, mainly after 2 PM. Otherwise a typical wind pattern for August should continue through the week with southwesterly winds backing southerly in the afternoon with the seabreeze, and veering westerly prior to sunrise with the landbreeze.

Thunderstorm chances should decrease Tuesday through Thursday as high pressure builds westward along the Gulf Coast. A stronger disturbance to our north should approach the Carolinas on Friday, potentially pushing a surface cold front into the area. This should lead to a larger coverage of showers and thunderstorms.

ILM WATCHES/WARNINGS/ADVISORIES

NC...None. SC...None. MARINE...None.