textproduct: Raleigh

WHAT HAS CHANGED

* The chance for storms has trended lower over much of central NC for the rest of today.

* Dangerous heat increasingly favored mid to late week, Wed-Sat.

* Convection chances still expected to return toward the weekend, but Fri has trended drier.

KEY MESSAGES

As of 250 PM Monday...

1) Slow-moving showers and isolated storms pose a localized flash flooding threat today. Persistent clouds will keep temperatures unseasonably cool over the Piedmont.

2) Heating up by Wed-Thu, but humidity levels low until Fri-Sat.

3) Continued hot this weekend, with increasing humidity levels and rain/convection chances.

DISCUSSION

As of 250 PM Monday...

KEY MESSAGE 1... Slow-moving showers and isolated storms pose a localized flash flooding threat today. Persistent clouds will keep temperatures unseasonably cool over the Piedmont.

The latest surface analysis shows that the initial weak frontal zone has pushed S and SW of central NC, situated across central and S SC back toward far SW NC. High pressure centered off the Northeast states stretches back through the Mid Atlantic coast, where a stronger front resides, well to our NE. The copious cloud cover and the cooler temps today (by as much as 5-12 deg) within a NE and E low level flow have helped keep MLCAPE in check, 500 J/kg or lower except near 1000 J/kg in our SE, where more holes have opened up in the clouds and where GOES imagery shows slightly greater vertical cloud extent. Precip today has been rather spotty thus far and focused mostly in the W Piedmont. While the column is more stable here, limiting updraft strength, the slow cell movement and steady downpours with these showers amidst high PW (~125% of normal) and lift contributions from isentropic upglide will still yield a risk for localized elevated rain rates and totals resulting in mostly minor flooding, particularly in urban areas and along smaller creeks. As the baggy mid level shear axis stretching W-E over N NC continues to ease southward through tonight, the rain chances will gradually slip south of our area and dwindle, leaving us mostly dry after midnight. Regarding temps, with the persistent clouds and precip in the NW Piedmont, temps will struggle to reach the mid 70s, while areas further E and S top out around 80 to the mid 80s. Expect lows tonight in the mid 60s to around 70 N to S.

KEY MESSAGE 2... Heating up mid to late week, but with low afternoon humidly levels.

A large and anomalously strong mid-level high, one which appeared to have tied the all-time observed record 500 mb height of 600 dam at Rapid City, SD last evening (and which may have broken it this morning if not for the void of 12Z RAOB data), will progress across the OH Valley and Middle Atlantic, then weaken and split through Fri.

At the surface, a weak high will extend from off the Middle Atlantic coast wwd and across the Carolinas early Wed, then yield to an Appalachian-lee trough forecast to develop across the Piedmont. The influence of those features, amid an increasingly strongly and deeply-heated boundary layer mid to late week, will likely cause afternoon/mixed dewpoints to decrease through the 60s F, with related RH mostly between 30-40%. So despite the hotter temperatures, with mid to upr 90s likely by Thu, heat index values should be tempered and remain mostly below 105 F (except for the Coastal Plain).

KEY MESSAGE 3... Continued hot this weekend, with increasing humidity levels and rain/convection chances.

While the aforementioned mid-level ridge will have split and weakened by the weekend, with a trough and nwly flow aloft likely to become established across the Middle Atlantic, its preceding subsident heating influence will linger until convection and/or a front eradicates the hot airmass. Neither appear probable until Sun; and there remains larger than average model spread regarding the amplitude of the troughing aloft and associated progression of a surface frontal zone into-through NC Sun into early next week. As such, hot conditions will likely persist for most/all of the weekend, with initially slight chances of convection on Sat increasing into the solid chance range on Sun.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

As of 730 PM Monday...

Ceilings will be challenging tonight through 15z/Tuesday. Expect mainly MVFR to IFR cigs to redevelop at most sites tonight by around 06z-08z, then persist through 13z-14z - before lifting out to VFR conditions. A few showers are possible tonight, mainly around KFAY.

Looking beyond 18z Tue, VFR conditions are likely to prevail through much of the week, although patchy sub-VFR stratus and fog may occur at FAY near daybreak Wed/Thu. Isolated late-day storms are possible Fri, then chances rise to scattered PM storms by Saturday and Sunday.

CLIMATE

Record High Temperatures:

July 16: KGSO: 99/1915 KRDU: 101/1887 KFAY: 103/1937

July 17: KRDU: 100/1887 KFAY: 102/1932

July 18: KGSO: 97/1986

July 19: KFAY: 101/2002

Record High Minimum Temperatures:

July 16: KGSO: 75/2024 KFAY: 78/1992

July 17: KGSO: 75/2025 KRDU: 80/1887 KFAY: 77/2019

July 18: KGSO: 76/1986 KRDU: 80/2025 KFAY: 77/2025

July 19: KGSO: 75/2016 KRDU: 80/1942 KFAY: 77/2023

RAH WATCHES/WARNINGS/ADVISORIES

None.