Extended Forecast
Valid: Fri 28 Sep 2018 06:00 to Sat 29 Sep 2018 06:00 UTC
Issued: Wed 26 Sep 2018 20:13
Forecaster: TUSCHY

SYNOPSIS and DISCUSSION

Thise extended forecast was issued mainly due to the risk of a potential (shallow) warm-core cyclone south of and over the S-Ionian Sea.

On Friday morning, an extensive cut-off will be centered over the Ionian Sea and further south. During the day a gradual turn to the NE is anticipated as this cut-off is forecast to get caught up by another strong upper trough over NE Europe, which approaches SE Europe during the night.

Despite this general agreement regarding the synoptic evolution, global models like IFS, GFS and ICON continue to diverge substantially regarding the exact structure of the deep vortex over the S-Ionian Sea which will be crucial regarding the track forecast of the intensifying LL vortex. ICON breaks the vortex into numerous smaller-scale mid/upper vortices, GFS shows a broadening upper vortex lifting NE towards Greece and IFS expecting an intense/deep vortex E of Sicily with a new upper vortex evolving to its NE (over Greece). Combined with different handling on the intensity of the LL vortex it is not of surprise to see an ongoing substantial model spread regarding the cyclone's track: IFS steering the LL vortex towards Sicily, whereas GFS and ICON indicating a drift to the N and NE (towards Greece), which is in line with the general lifting trend of the upper vortex (all track forecasts until Saturday 06 UTC only).

Latest WV data highlights a significant tropopause fault with the intrusion of very dry mid/upper air along the W/NW fringe of the cut-off (sampled well by Brindisi, 26th September at 12 UTC). Until Friday this dry air and attendant upper level jet maximum are forecast to circle the deep vortex and both will push NE towards Crete and the Aegean Sea, cutting off a pool of warm/moist and unstable air to the west (beneath the mid/upper vortex). SBCAPE will be in the 500-1000 J/kg range and supportive for vigorous convective activity. However, upper divergence starts to weaken over the S-Ionian Sea as the upper jet pushes NE and away from the main vortex. With decreasing baroclinicity and shear, the main driver of the LL vortex strength becomes the convection, which starts to organize during the forecast period (here organized convection means convective banding and not rotating updrafts!). Abating divergence could be a crucial point regarding a slow-down of convective activity and attendant organization into convective bands.

Latest phase diagrams remain somehow reluctant regarding the speed of the transformation of the thermal structure, but still indicate a shallow warm-core structure (excluding IFS, which continues to advertise a deeper warm-core anomaly). Uncertainties regarding the intensifiaction rate and strength of the LL vortex depend on:

- the potential of better upper-level ventilation to the NE into the upper jet which also depends on the final structure of the upper level vortex

- how broad the LL vortex will be with a smaller vortex susceptible to more rapid intensity changes (up and down)

The first point is not yet supported by all global models due to the different geometry of the geopotential height fields. The second point seems to be crucial as ICON and GFS prefer a broader LL circulation which would need more time for intensification, whereas IFS already starts the forecast with a compact vortex which could intensify much faster. ASCAT data will give more and helpful input into that topic as the vortex forms but for now we lean more towards ICON/GFS (slower intensification rate) given the large nature of the upper vortex and the persistent forecasts of both models during the past few days regarding the size of the LL vortex (plus abating divergence).

Otherwise the background environment remains favorable with SSTs of 26-28C (above climatology), weak DLS next to the evolving LL vortex and deep moist profiles with elongated thin CAPE profiles.

Mentioned uncertainties preclude confined areas with higher probabilities for now but the main risk will be excessive rain, severe wind gusts (mainly driven by the ageostrophic response to the deepening depression) and isolated tornadoes. Further model data will be evaluated to determine certain areas with an higher severe risk, especially if a potential landfall becomes clearer.


Aside from that cyclone two more areas with organized convection exist over:

Crete and the Aegean Sea:
A northward surging upper jet and PVA ahead of the upper trough create supportive conditions for lift over a broad area. Low-tropospheric moisture increases from the south with MLCAPE aoa 1kJ/kg affecting the highlighted area. Straight and elongated hodographs indiacte a risk of organized and splitting multicells/supercells with DLS in excess of 20 m/s and enhanced SRH-3. A broad level 1 was issued for this area due to ongoing uncertainties (e.g. where DMC could be suppressed by the deep dry slot).

Baltic States and areas east:
A deep low races east during the start of the forecast. The cold front passage could serve as focus for the development of a forced line of deeper updrafts, which could bring some momentum down from the 850 hPa / 700 hPa layers with wind speeds of 25 m/s / 35 m/s. A few severe wind gusts would be the result and an upgrade could become necessary along/ahead of the cold front. The main limiting factor right now is weak SBCAPE as BL moisture starts to mix out beneath weak mid-level lapse rates.
A few post-frontal thunderstorms are forecast over the Baltic States and offshore later the day which stay sub-severe.

Sporadic lightning is possible along the coast of CNTRL Norway.