© Nicholas J. Verge
CONVECTIVE OUTLOOK AND DISCUSSION FOR WESTERN AND NORTHWESTERN EUROPE, FOR
PERIOD 0000Z SATURDAY JUNE 05 2004 TO 0600Z THURSDAY JUNE 10 2004
1. SYNOPSIS
There is moderate confidence of the potential for widespread strong
convective storms across western and northwestern Europe (exclusive of all
but southernmost Scandinavia) from 00Z Saturday June 05 2004 and for
perhaps next five days (or more)
There is moderate confidence in the potential of some of these convective
storms being severe or very severe across parts of Wn and NWn Europe.
Extent of threat area reflects inter-NWP model and model inter-run
variation
2. AT RISK REGIONS/NATIONS IN DETAIL
2.1 SEVERE CONVECTIVE STORMS
There is low to moderate confidence of a slight or greater risk of
potentially severe or very severe convective storms occuring over the
following regions/nations during the above period. Extent of threat area
reflects inter-NWP model and model inter-run variation:
# Wn and NWn and Nn France;
# England, particularly, central and Sn;
# Wales, particularly Sn;
# Belgium, all;
# Netherlands, all;
# Wn Germany
# Demark, particularly Sn
# Sn Sweden (but currently low risk)
# Sn Norway (but currently low risk)
2.2 STRONG, BUT MOSTLY NON-SEVERE CONVECTIVE STORMS
There is low to moderate confidence of a slight or greater risk of strong,
but otherwise mostly non-severe convective storms across the following
nations/regions during above period:
# All of Iberia (Portugal and Spain)
# All France
# All England
# All Wales
# All Ireland (Irish Republic and Northern Ireland)
# All Belgium
# All Luxembourg
# All Netherlands
# All Denmark
# Western Germany, possibly all Germany later.
# Far Sn Sweden (but currently low risk)
# Far Sn Norway (but currently low risk)
# Sn Scotland, perhaps all Scotland (but low risk)
3. BASIS/RATIONALE
Recent output from major NWP models notably UKMO GM and particularly GFS,
suggest that many if not all of the requisite features, upper air, low
level and surface, thermodynamic and dynamic, are forecast to be in place
for strong and/or severe thunderstorm development across Wn and NWn Europe
(exclusive of Scandinavia, by 00z Saturday June 05 (but perhaps a little
later or a little earlier). Although there are difference between these
models and inter-run variation the general scheme is as follows:
3.1 UPPER AIR PATTERN
During the latter of the week beginning May 31st an amplifying, sharpening
and perhaps increasingly negative tilted longwave upper trough/coincident
thermal trough is forecast to dig southeastwards towards Iberia from
deepening upper low/vort max south of Iceland, whilst upper longwave ridge
and coincident thermal ridge amplifies northwards across Wn Europe from NW
Africa/Iberia. As a result thicknesses are forrecast to build across Europe
towards latter part of weak and weekend, such that by weekend 564dam
thickness contour will lie near or over British Isles/Benelux.
3.2 LOW LEVEL PATTERN
At surface, a weak frontalysing cold front is expected to be located from a
low south of Iceland/west of Scotland, southwards across Wn British Isles
through Biscay to west of Iberia. Front is likely to become increasingly
slow moving and may begin waving as upper air disturbances move over it.
Azores ridge initially present NE-SE across British Isles is expected to
decline across BIs, leaving a col over BIs between Azores ridge to SW and
breakaway high over N Sea/continental Nn Europe/Mediterranean
Combination of amplifying and perhaps increasingly negative tilted E
Atlantic upper trough and longwave ridge is FC to cause warm air advection
from North Africa across Wnmost Europe from late Friday, ahead of
approaching cold front. Resulting high theta-E plume and associated
coincident shallow prefrontal surface trough(s)/surface convergence zones
will extend northwards reaching southern UK by Saturday/early Sunday.
Some runs suggest that two distinct plumes may develop, a narrow western
one extending N from Wn Iberia and another broader across central Iberia
and France. If this occurs, the western one, will likely become elevated as
it approaches British Isles, and subsequently extend N across Eastern
Ireland into Scotland. Eastern Plume will likely extend to surface and
extend northwards across France into southern central UK, the Benelux
countries.
3.3 THERMODYNAMICS
Below high theta-E plume Tds are fc to increase towards the high teens,
possibly low 20’sC across France. Highest Tds are expected within surface
convergence zones as a result of moisture convergence. Significantly,
derivation of theta-E plume from NW Saharan Africa suggests the high Td
airmass may be well-capped by an EML. This combination, together with upper
air ELRs that may be somewhat greater than seasonal norms, may result in a
highly conditional unstable lower atmosphere (a “loaded gun” scenario)
across Wn and NWn Europe by Sunday afternoon, during Monday and into
Tuesday. During this period conditional upon sufficient deep BL moisture
being present and high insolation occuring, instability will be high, and
locally very high. CAPE values are likely to be significantly in excess of
1000J/kg across much of France, Iberia, possibly England and Wales, perhaps
approaching 2500J/Kg in some areas eg Nn France/central England in
particular. 1000-500hpa Lifted indices significantly less than 0 are
expected across the same areas and may be as low or lower than -10C
locally. Should these CAPE aand LI values be attained, the potential for
explosive convection would exist in these areas.
3.4 DYNAMICS
By 00z Sunday, at high latitudes westerly upper (300-200hPa level) jet max
south of upper low is FC to be strongly diffluent across upper longwave,
and to split into northern and southern streams. The northern stream is FC
to swing north, becoming Sly and for its speed max of 50-60Kts to
approach/overspread the Wn British Isles. The Sn stream will swing south
and round base of LUT and then swing N across Iberia. Significantly, along
forward eastern flank of LUT, there are indications that mid-upper level
(ca 500-200hPa) jet will develop and push north across Wn france and during
Sunday overspread across the UK. Thereafter this jet is currently expected
to strengthen during Monday and Tuesday. There are indications in some
model runs that this mid-upper level jet will likely contain several jet
maxes of the of order of 10-20Kts above background. Importantly, there are
also indications that one or more minor Sly low-level jets will develop
along forward flank of upper longwave trough and travel northwards across
British Isles during Sunday. LL jet maxes of the order of 20-25Kts are FC.
Consequently 0-1Km and 0-6 Km deep layer and total sub-trop shear has the
potential to be significant across Wn and NWn Europe, particulalarly over
the British Isles, especially from late Monday and during Tuesday. Mid and
upper level shear is forecast to unidirectional SWLy or Sly. However, at
LLs the directional component is likely to be more variable, governed by
the location of subtle surface troughs, ridges and shallow (thermal) lows.
On some runs the ridge FC to be declining over the British Isles/N Sea is
FC to evolve into a dicrete high over the N Sea and central continental
Europe. If this occurs, surface flows across N and NWn France, Benelux and
UK are likely to become more-backed SEly enhancing directional shear/SRH.
LL directional shear/SRH is also likely to be significantly also/further
enhanced in immediate vicinity of En flank of surface troughs and their
included convergence zones, close to topography, and as a result of sea-
breezes.
4. DMC MODES AND THEIR ASSOCIATED POTENTIAL SEVERE WEATHER THREATS Both boundary layer rooted, and elevated deep moist convection are
predicted during this WAA event. Hybrids of the two modes are also
possible.
In general, initiation of BLR DMC will likely be conditional on sufficient
strong insolation occurring in the absence of significant cloud cover.
Otherwise and in addition, it is most likley where LL/surface theta-E
“hotspots” form as a result of strong moisture convergence. Experience
suggest that in a Sly or SWly flow these are likely to at promontories
along the N/NW coast of France and NE coast of England (Cherbourg and Brest
Peninsulas, East Anglia and Lincolnshire Peninsulas) where warm high Td air
is funnelled by a combination of a cooler stabler BL over the sea and by
onshore sea breezes. Equally, BLR DMC may also develop anywhere within
areas of strong LL moisture convergence within prefrontal CZs and
particularly where these CZs cross coastlines (for reasons above) and
upland areas (Welsh Hills, Moors SW England, Pennines, hills southern
England, N and NW France, Massif Centrale, Ardennes, Vosge, Pyrenees, hills
central Iberia), where orographic forcing will be greatest.
Elevated DMC is most likely across NWnmost and Wnmost Europe, and will
likely be triggered by upper air cooling after sunset destabilising
elevated high theta-E plumes. Destabilisation of such plumes may also occur
as a result of orographic forcing where they move over high ground.
4.1 BOUNDARY LAYER-ROOTED DMC
Strong and/or severe, possibly very severe convective storms are forecast
for Wn and NWn Europe from 00z Saturday June 5. DMC leading to severe
convective storms is forecast to develop when and where the high theta-
E/high Td plumes are overspread by DLS. Current indications are that this
will most likely occur across NW/Nn France, Wales, and England. Where this
occurs, well organised multicell cluster storms can be expected, posing a
risk of prolonged high straightline outflow winds and large hail. However,
if environmental vertical vorticity is strong within the inflow layer and
can be deformed to become horizontal vorticity within such storms, DLS may
be sufficient to initiate the development of strong, persistant
mesocyclones and eventually supercells through cell-splitting. Mesocyclonic
storms/supercells may also develop directly, as a consequence of the
ingestion high inflow layer SRH. This is most likely to be possible where
the LL flow becomes backed SEly for synoptic or mesoscale reasons, as
described above. If and where mesocyclonic storms/supercells form, the
risk of prolonged severe weather will be great, with the additional
potential of the sustained production of very large hail (>5cm diameter),
and perhaps tornadoes. Alternatively, fast moving linear MCS’s may evolve
out of groups of initially isolated multicell storms. High straightline
winds (as downbursts and microbursts, possibly as larger scale derecho
events) and severe hail being the main threats from such organised storm
systems.
Outside regions of significant DLS, DMC will most likely be in the form of
single cell “pulse” type and/or poorly organised multicell cluster storms,
possibly developing into linear slow-moving or quasistationary MCSs,
backbuilding in and along convergence zones, and/or over and downwind of
high ground. The majority of these storms will likely present only a low
risk of producing severe weather forms. However, there is risk that severe
storms may also occur where instability (=CAPE) is high (ca >1000J/Kg) and
consequently parcel buoyancy/updraught velocity also high. Brief falls of
severe hail could potentially be a product of such storms, together with
short periods of high-straightline outflow winds, possibly as
downburst/microburst events, particularly if BLR convection is high-based,
and above dry warm sub-cloud layers. Additionally, where LL flow becomes
marked backed strong SRH in the inflow layer, may result in mesocyclone
formation, posing an greater risk of more-prolonged severe weather,
including sever hail and high straightline winds and possibly weak
tornado/landspout tornado formation. However, in the absence of significant
DLS, any mesocyclones are likely to be short lived. This is most perhaps
most likely along the eastern margins of LL/surface convergence zones.
4.2 ELEVATED DMC
If DMC convection develops from elevated high theta-E plumes the most
likely severe weather form is high straightline downburst/microburst winds,
particularly if dry sub-plume/cloubase layers are present. However, in
areas of high DLS any elevated well-organised multicell storms will also
have the potential to produce severe hail. If the elevated inflow layer
exhibits either high SRH or vertical vorticity, elevated
mesocyclones/supercells may develop by same process as for BLR storms. If
this occurs large hail may be produced by such storms. However, conversely,
unless the base of such elevated mesocyclonic storms/supercells is low, the
risk of any tornadoes from such storms is minimal.
4.3 ALL DMC MODES
All convextive storms will have potential to be well-electrified and to
produce high precipitation rates and totals, leading to local flash
flooding in urban areas and in impermeable drainage catchments.
5. DISCUSSION
The above is a review of possible consequences of FC warm air advection
event across Wn and NWn Europe. It is likely that some or all of the above
will not occur.
Presently there is moderate confidence that strong BLR convective storms
will affecting Wn and NWn Europe from 00z Saturday into the middle part of
the following week, perhaps beyond as systems migrate eastwards into
central Europe. At time of writing with release of the output from the June
01 00Z 06Z 12Z 18Z and June 02 00z inis of GFS, the most likely locations
of any strong storms looks like being across Nn France and British Isles,
particularly England and Wales. There, severe convective storms may result
in these areas if high deep layer shear overspreads these regions. Whether
this occurs will be highly conditional on development of upper air pattern,
notably amplification and eastward overspreading of LUT and associated
eastward migration/strength of upper jets over thigh theata-E plume. Also
wrt to one scenario for supercell development, significant backing of
strong LL winds occuring. This is likely to also be highly conditional, but
this time on the development and positioning of synoptic and shallow
emphemeral thermal lows, prefrontal convergence zones and synoptic ridges
and anticyclones.
At time of issue (0600Z June 02 2004) the development of severe storms
looks most likely to happen during Sunday, and/or Monday and/or Tuesday
an/or Wednesday and afternoon/evening across England, Wales, Nn and NWn
France.
Members are advised to be alert for updates on this developing potentially
severe weather event via Eurochase and by visiting the website of the
European Storm Forcast Experiment (ESTOFEX) - http://www.estofex.org/
Comment on, critism of and contributions to this outlook/discussion are
welcomed.
Issued by: Nicholas J. Verge 0600Z June 2, 2004
© Nicholas J. Verge
Footnote: Indications are that this potential convective storm event, could
produce the worst severe storms to affect the Bristish Isle for seven years
or more.
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