Mesoscale Discussion

Mesoscale Discussion
Valid: Sat 09 Sep 2023 08:00 to Sat 09 Sep 2023 11:00 UTC
Issued: Sat 09 Sep 2023 08:32
Forecaster: TUSCHY

Although not completely in our area of interest, an update regarding the convectively driven depression over the Gulf of Sirte will be issued in this MD.

The vortex, with the international name "DANIEL", is analyzed just shy south of 34N and around 18/19E with a constant drift to the E/ESE. During the night, the mentioned wedge of more humid air finally encircled its center (cloud system center, CSC) and assisted in an more substantial/widespread increase in DMC activity. Lightning activity over the past 12h was mainly confined to the prominent convective band in the W/N quadrant of the depression but an uptick in activity near the center was noted in the past 3 hours.

Regarding the pattern for most of the night, DANIEL featured temporarily flaring convection over the W part of the center and also in general along the W quadrant (within the more persistent convective band). Since 3Z however, a persistent curved band finally became anchored atop the CSC, which can be used for the Hebert-Poteat classification.

Cloud tops in the -50/-55 C range support deep updraft growth, probably up to the 300/250 hPa layer, which is important regarding shear magnitude. Area averaged soundings give roughly 35kt 200-850 hPa shear with slightly lower values based on satellite observations. Adding the easterly motion of the vortex results in low to moderate shear values atop DANIEL (convection sheared to the NE) so steady structural improvement atop SSTs of 26-28 C is forecast as subtropical systems in general are more resilent against shear.

Finally the mentioned short wave now passed Tripolis/Misrata and drying dark stripes in WV imagery point to the developing (but weak) coupled jet configuration. Upper divergence/mass evacuation is in line with flaring convection over a broad area of DANIEL's vortex.

The past few ASCAT scans showed a tightening vortex with winds just shy below subtropical storm strength but given current banding and general structural improvement, I would go with a ST 2.5 and would expect subtropical force wind gusts next to the center. However with ongoing improvement of the banding (maybe pushing a bit more S and covering the complete CSC) within 1 degree of the CSC, we're probably already pushing to ST 3.0 and hence in the solid subtropical storm range of 40-50 kt later today. The next ASCAT scan will give more details on that.

NWP guidance converged to a landfall near Benghazi until 06Z with gale-forced gusts over the Gulf of Sirte and coastal areas. Rainfall becomes a serious issue beyond 06Z.

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