Storm Forecast
Valid: Mon 16 Dec 2013 06:00 to Tue 17 Dec 2013 06:00 UTC
Issued: Sun 15 Dec 2013 21:12
Forecaster: TUSCHY

A level 1 was issued for Crete mainly for excessive rainfall amounts. An isolated waterspout event is possible, too.

SYNOPSIS

A rather high AO index with the polar vortex still placed over Greenland and E-Canada keep intense band of westerlies/southwesterlies in place over N-Europe. Downstream ridge affects most of S/C Europe. The main progressive features will be compact cold-core lows over SE-Europe. One leaving the Ionian Sea to the E around noon with a constant weakening trend forecast and another one approaching the Adriatic Sea during the late afternoon hours with a constant motion to the south.
Another progressive feature will be a developing mid-layer wave, which crosses Scotland, Norway and Sweden during the day from SW to NE.

DISCUSSION

... Crete and surroundings ...

A weakening and eastward moving cold-core low crosses southbound sliding low-tropospheric convergence zone/moisture boundary. Modest BL moisture resides ahead of that boundary, so a temporal overlap of some mid-layer forcing, moisture and instability exists over the area. Regarding CAPE, lowering moisture content behind the front will be partially offset by cooling mid-levels (500 hPa around -24 °C). Effective PWs will stay aoa 15 mm with DLS mainly in the 10-15 m/s range (highest values displaced from best CAPE). Nevertheless, 0-3 km CAPE increases to 150 J/kg over the area of interest. The main concern will be slow moving convection with augmented precipitation efficiency ... further increased by upslope component over Crete. Heavy to excessive rain is possible. Another concern is the presence of numerous shear and/or convergence zones mainly north of Crete. Combined with aforementioned LL CAPE, waterspouts will be another hazard. The overall risk diminishes from north to south during the day with strongest convection displaced to the south of Crete during the overnight hours.
We would not be surprised about a N-S aligned line of convection over the far W-Aegean Sea during the late night hours. With CAA atop the warm Aegean Sea, LL CAPE in excess of 100 J/kg is forecast, so isolated waterspouts are possible. Quasi-stationary nature of those storms could also offer high rainfall amounts. With ingredients becoming worse betimes, no level area was added with low-end lightning area expanded far north.

... NE-Scotland to SW-Norway ...

A developing wave exits Scotland to the NE during the morning hours and approaches SW Norway around noon. Rich prefrontal BL moisture becomes trapped in the widening warm sector, although warm mid levels keep the stratification stable. Of interest will be the developing tropopause fold along the tip of that wave, which overspreads the NE-ward racing cold front/occlusion. A confined time-frame between 6-12 Z exists, where enhanced convection could evolve along the N-part of that front between NE-Scotland and SW-Norway. Shear would be more than adequate for severe or damaging wind gusts as 850 hPa winds exceed 25-30 m/s. Later on, CAPE diminishes over SW-Norway as onshore moisture decreases. Impressive forcing could keep a forced line of probably broken convection alive for a few more hours/well inland. Ongoing uncertainties with track and strength of that wave and low confidence in an organized/closed line of deep convection preclude any level area for now.

Otherwise, active marine convection is forecast over the Norwegian Sea. Sporadic lightning is possible and a broad lightning area was issued.