Extended Forecast

Extended Forecast
Valid: Tue 01 Aug 2017 06:00 to Wed 02 Aug 2017 06:00 UTC
Issued: Sun 30 Jul 2017 18:01
Forecaster: TASZAREK

*** This extended forecast was issued to highlight a possibility of severe weather outbreak in the corridor from CNTRL France up to N Poland, an update to level 3 is possible ***

Please note that due to a lead time of 2 days, 50% lightning probability line was not drawn.

SYNOPSIS

On Tuesday, a warm, moist and highly unstable tropical airmass will be covering most of SW, CNTRL and S Europe. In the corridor from SW France, through Benelux and up to Lithuania, a quasi-stationary frontal boundary will be dividing aforementioned airmass from much cooler polar air over NW France, British Isles and Scandinavia. Due to decent horizontal thermal gradient, a strong jet will be stretching from Iberian Peninsula up to W Russia, providing a broad overlap of high shear and moderate to high CAPE environment. Whole circulation will be driven by a large ridge over SE Europe and low over N Atlantic. Thunderstorms capable of producing extremely severe weather including tornadoes, very large hail, damaging wind gusts and heavy rain are predicted to initiate on the warm side and along the boundary in the early afternoon hours.

DISCUSSION

...corridor from CNTRL France up to W Lithuania...

Dew points exceeding 20C develop over E France, Germany and NW Poland as a result of a strong moisture pooling and evapotranspiration. Steep mid-level lapse rates from a source region of NW Africa and Iberian Peninsula move with a SWerly flow and overlap with moist boundary layer. An additional ingredient of a strong diurnal heating combined with two other factors create a healthy ML CAPE of 1500-2500, locally up to 3500 J/kg. Broad jet streak with 20-25 m/s at 700 hPa overlaps with a large portion of instability zone providing an impressive WMAXSHEAR (a product of square root of two times CAPE multiplied by DLS) exceeding 2000 m2/s2. Majority of unstable air in this zone benefit from 25-30 m/s DLS while NE Germany and NW Poland lies under 20-25 m/s MLS, and 10-15 m/s LLS. Although CIN values will be enhanced, a passage of the shortwave trough is expected to destabilize the airmass and provide CI. Increased CIN values may support a discreet convective mode at the early stages (early afternoon hours). Given such kinematic and thermodynamic field, these storms may quickly evolve into isolated supercells capable of producing large to very large hail and tornadoes. The most prominent environment for these covers a corridor from CNTRL Germany up to N Poland. Some NWP models predict initiation quite early which may slightly decrease severe weather potential of these storms, especially in terms of tornadoes which are more likely in the late afternoon hours when LLS is higher. In the late afternoon hours two NEwardly moving MCSs are plausible, one over N Poland and second in CNTRL Germany. Given decent instability and 25 m/s flow at 700 hPa, squall lines with bowing segments capable of producing damaging wind gusts are plausible. A derecho event cannot be ruled out.

This forecast has few limitations that during the next 2 days may strongly modify the scenario. These concern:
1. Timing and the amplitude of the shortwave which will be crucial in terms of CI, its placement and convective mode. This is probably the most important factor which should be carefully followed in the next model updates.
2. CIN values that may inhibit convective activity
3. LL moisture that may be overestimated in NWP models and result in a lower instability.
4. Cloud cover in the morning hours, which may limit diurnal heating, decrease instability and complicate CI.


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