Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2nd, 2014

Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2nd, 2014Nathan MarsiliNWS Northern Indiana2014 GLOM Workshop

SYNOPTIC SETUPPHASE 1

300 MB700 MBFEBRUARY 1/1200Z300 mb Right entrance region of upper jet from eastern Iowa into northeast Illinois.

700 mb Positively tilted upper flow, slightly backed mid level flow inducing low/mid level WAA ahead of trough. Widespread precip as a result of mid level trough, WAA forcing.2SYNOPTIC SETUP

300 MB700 MBFEBRUARY 2/1200Z

300 MB 700 MB300 mb favored area of upper divergence in right entrance region shifted eastward into northern Illinois/northern Indiana.

700 mb Mid level flow now oriented more parallel to isentropes. Tightly backed mid/upper level baroclinc zone raises questions about frontogenetic response for forcing latter portions of the event.3HEADLINES*A 2-phased winter storm impacted area Feb1-Feb2.*These different storm phases led to two distinct areas of warning or near-warning criteria accumulations:

Wx Story - Forecasts locally focused on phase 1 for warning or near warning criteria snow. Precip type issues and relatively short duration of thermal profiles supporting all snow coincident with forcing lead to only minor accums forecast across southeast half.

Storm total snow Two swaths of warning or near-warning criteria snowfall. The first of these areas was across far northwest Indiana and southern lower Michigan with phase 1/WAA induced portion of storm, and then across far southeast with phase 2 frontogenetic/deformation forced portion. Narrow corridor across central portions of forecast area with little or no snow accumulation.

4EVENT OVERVIEW00Z-12Z KIWX 0.5 Base Reflectivity

Base reflectivity loop showing the longeviity of banding.5EVENT OVERVIEW00Z-12Z KIWX 0.5 Base Reflectivity

Base reflectivity as in previous slide except with delineation of approximate rain/snow line.6EVENT OVERVIEW

Narrow band of heavy snowSatellite imagery from Feb 3 gave indication as to location of narrow zone of higher snow accums from central7EVENT OVERVIEW

NAM QPF VALID FEB02 06-12Z12Z FEB1 NAM 9-hr QPF 06Z-12Z (F18-F24)00Z FEB2 NAM 6-hr QPF 06Z-12Z (F06-F12)Model forecasts exhibited quite a bit of flip flop in placement.

Left Feb1 12Z NAM F18-F24 QPF with bulk of heaviest precip southeast of observed heavy snow area after 06Z. Right Feb2 00Z NAM F00-F06 QPF indicating northwest shift to QPF.8EVENT OVERVIEW

Forecasters struggled with phase 2 of system large run to run variability in southward progression of mid/upper level frontal zone.

Precip type issues also complicated phase 2 of snowfall forecast across southeast portions of forecast area.

Forecast thinking: Small window of time with profiles conducive for snow and presence of mid/upper level frontogenesis would limit snowfall amounts to less than 2 inches across the southeast.

Upstream sheared waves in southwest flow aloft critical in determining placement of mid level baroclinic zone.

What were some of the important factors leading to the banding toward the end of the event across the southeast portions of forecast area?

INGREDIENTS METHOD

Wetzel and Martin (2001) developed a qualitative forecast approach assessing 5 basic ingredients to heavy snowfall forecasting:

Forcing for ascentInstabilityMoistureEfficiency (snow growth)Thermal profiles (top-down considerations)

PVQ set to 0 if:Q-vector divergence OREPV positivePVF set to 0 if:F-vector divergence OREPV positive

Remainder of case study will focus on ingredients 1 and 2 listed from Martin and Wetzel, forcing and instability.PVQ defined by Wetzel and Martin is product of Q-vector convergence and negative EPV (defined as 0 elsewhere). After 2001 journal article, Wetzel/Martin developed PVF to capture effect of frontogenesis. They used product of magnitude of frontogenesis and negative EPV, but for this study have replaced frontogenesis with divergence of F-vector to better capture thermally direct circulation.

10INGREDIENTS METHOD

Pros:

Quick way to assess potential areas for further investigation.

Flexibility for number of different situations.

Cons:

Does not distinguish between types of instability.

Not useful when model forecasts are poor.

Qualitative in nature.

Heavy/banded snow can occur in higher stability regions.

Not a substitute for more in-depth analysis.

Magnitude of PVQ/PVF difficult to interpret (model resolution dependence).

Example from Wetzels ingredients page to show what the products look like.11INGREDIENTS METHOD

Ingredients method utilized for Feb 2 case, calculating PVQ and PVF.

Slight modification made to PVF for current case to include F-vector divergence instead of frontogenesis to better capture model vertical motion due to frontogenesis.

Calculations of PVF and PVQ integrated for 50 mb increments from 850-500 mb and plotted on plan-view. (only layers of mean RH >= 80% plotted).

00Z NAM 850-500 MB PVQ/PVF

PVF F06Valid 06ZPVF F09Valid 09ZPVQ F06Valid 06ZPVQ F09Valid 09ZFousing on the period of heaviest snow 06Z-09Z, above are PVF and PVQ from 00Z (F06-F09). PVF shows a weak signal across southern portions of the area (not fully shown due to contour interval). PVQ shows much stronger signal in area of heaviest snow, particularly by 09Z.1300Z NAM 850-500 MB PVQ/PVF

PVQ F09Valid 09ZPVF+PVQ F09 Valid 09Z PVQ F09 Valid 09ZThe addition of PVF and PVQ shows a better correlation to heavy snowfall at 09Z, although 00Z NAM diagnostics appear to be too slow in generating the strongest forcing.1400Z NAM CROSS SECTION

00Z NAM Omega (contours), Q-Vector convg (color-filled) F09/Valid 09ZCutting a cross section through the PVF+PVQ signal, perpendicular to the thickness countours, shows strongest Q-vector convergence signal mainly in the 600:500 mb layer which lines up well with model stronger UVM fields.15INGREDIENTS MAP EXAMPLE

00Z FEB2 NAM F009/V09Z550-500 mb negative EPVg (color-filled) Q-vector convergence (dashed yellow)

Now that layer of interest has been somewhat narrowed, a smaller 50mb layer can be inspected with Wetzel/Martins ingredients maps for more detail. 550:500 EPV and 550 Q-vector convergence show strong overlap in approximate area of heavier snow.16INGREDIENTS MAP EXAMPLE

00Z NAM F009/V09Z550-500 mb PVQ

PVQ diagnostic (product of Q-vector convergence and negative EPV) shown here.1700Z NAM Q-VECTOR PARTITION

00Z FEB2 NAM Qs vector and Qs convg. F09/Valid 09z00Z FEB2 NAM Qn vector and Qn convg F09/Valid 09zAbove analysis showed Q-vector convergence signal correlates well with heaveist snow. To better understand what processes might be important in creating this signal, Q-vector partitioning performed with Qn and Qs. Both Qn and Qs provide positive contributions in the area of heaviest snowfall. Qn = geostrophic frontogenesis magnitude while Qs = geostrophic rotational frontogenesis. Both Qs and Qn exhibit fairly strong convergence in area of heaviest snowfall. Qn orientation appears to follow fairly closely generaal orientation of banding, while synoptic forcing appears to have had additive effect in enhancing mesoscale response.18Q-VECTOR PARTITION

Martin 1999Martin (1999) linked Qs convergence to the development of thermal ridging in occluded/TROWAL portion of cyclone.

Qs vectors pointing with cold air to the left represent counter-clockwise turning ofthickness pattern, and with cold air to the right represents clockwise turning of pattern.

Qs convergence associated with larger scalesynoptic forcing through 1) vorticity adv. and 2) shearing deformation

To gain further understanding from Qs convergence signal, reference to Martin 1999 which linked thermal ridge development in TROWAL to strong Qs convergence. Geostrophic voriticity advection by thermal wind and shearing deformation are key factors in Qs convergence. Both likely at play in this case. Qs convergence also likely explains development of 600-500 thermal ridge in this case between 06Z and 12Z.19Q-VECTOR PARTITION

00Z FEB2 NAM 600-500 mb Qs convg 550 mb temps F06/V06ZF09/V09ZF12/V12Z00Z NAM F06-F12 showing 600-500 Qs convergence and development of upper level thermal ridge. (rotational frontogenesis)20Q-VECTOR PARTITION

Lack of consistent F vector full wind frontogenesis signal to heavier snow bands. (poor model resolution of full wind field?)

Strong upper level Qs and Qn convergence signals from the 00Z NAM in approximate area of heavier snow bands.

Geostrophic frontogenesis (Qn) appeared to correlate well to orientation of the heavier snow band (flow characterized by large deformation mag)

Rotational frontogenesis (Qs) may have served important role in prolonging presence of stronger mid/upper level baroclinicity by changing orientation of mid/upper level baroclinic zone.

Thermal ridge development/rotational frontogenesis increased temporal overlap between upper forcing and favorable frozen P-type thermal profiles.

Synoptic scale processes (Qs) appear to have maximized mesoscale response.

MODEL/OBSERVATION TRENDS

00Z FEB1 NAM 700-500 mb vorticity, 600 mb wind speedF33, Valid 09Z/FEB1F33 from 00Z FEB1 NAM. 600 mb jet is weaker and displaced further south.22MODEL/OBSERVATION TRENDS

00Z FEB2 NAM 700-500 mb vorticity, 600 mb wind speedF09, Valid 09Z/FEB1F09 from 00Z FEB2 NAM. 600 mb jet is stronger and dispalced northward. More pronounced shearing vorticity as forecast time draws closer.

23MODEL/OBSERVATION TRENDS

00Z FEB1 NAM 550 mb temp/deform. F33/Valid 09Z00Z FEB2 NAM 550 mb temp/deform. F09/Valid 09ZF33-F09 NAM evolution showing northward adjustment to strongest upper level front and stronger deformation near this baroclinc zone as forecast distance gets smaller. 550 mb thermal ridge also evident likely aiding in northward displacement of front.24

MODEL/OBSERVATION TRENDS0.5 KIWX Base Reflectivity 06Z06Z FEB2 NAM 600 mb F-vector dvg F00/Valid 06Z06Z at initialization showed fairly strong correlation between upper level frontogenesis and returns. Full wind frontogenesis seems to do a respectable job here.25

MODEL/OBSERVATION TRENDS0.5 KIWX Base Reflectivity 09Z06Z FEB2 NAM 600 mb F-vector dvg F03/Valid 09ZHowever, by 09Z, the 3-hour forecast from NAMs 600-500 mb F-vector divergence not as well correlated to banding, especially northern band. Northern band appears more linked to geostrophic frontogenesis, indicative of importance of synoptic scale processes.26

CSI ASSESSMENT

Uniform southwesterly geostrophic flow.

Deep saturated/near-saturated conditions.

Strong vertical wind shear/weak stability.

Observed bands spatial and temporal scales fit slantwise conceptual model.

Orientation parallel to thermal wind.

Favorable location, south of jet streak.

Large scale ascent.

INSTABILITYCSI checklist seems to work out well for this case.27

CSI ASSESSMENT

INSTABILITYWeismueller/Zubrick 199800Z NAM KFWA fcst sounding F09/Valid 09zNAM 9-hr forecast very similar to another CSI case from Weismueller and Zubrick in 1998. Low level stable layer, elevated layer of near moist adiabatic lapse rates. Pronounced unidirectional southwesterly wind profile with good deal of speed shear above stable layer.28

CSI ASSESSMENT

INSTABILITY00Z NAM KFWA Forecast Sounding (F09/Valid 09Z) Cross-hair signature shows up with NAM 650-500 mb omega matching well with dendritic growth zone.29

AAINSTABILITY0856Z 0.5 KIWX Base Reflectivity30AA

INSTABILITY06Z NAM EPVg (color-filled), omega (green), theta-e (white) F003/V09ZCross section for F03 from 06Z NAM showing large area of upper level instability from 650-500 mb. No real evidence of convective instability except across far northern portions of the cross section where moisture profiles were too dry for significant precip. Presence of negative EPV, high RH, and theta-e profiles suggest slantwise convection.31

EPVg Calculations for 2/2/14 Banded CaseLarge run to run model variations in distribution from 36 to 6 hours before heavy snow.

Some utility in the 6 to 12 hours preceding heaviest snow (especially < 6 hours).

Provided some indication to the potential of heavy banded snow, but not very helpful in banding location more than 6 hours before heavy snow.

Models slow to latch onto rotational frontogenesis aspect of event, and thus inconsistent depiction of frontal instability.

Small model initialization errors with mid/upper level wind field will hurt reliability of interpretation.

INSTABILITYAbove represent challenges in using EPVg operationally.32

INSTABILITY00Z FEB1 NAM 550:500 Negative EPV F33/V09Z12Z FEB1 NAM 550:500 Negative EPV F21/V09Z00Z FEB2 NAM 550:500 Negative EPV F09/V09ZEvolution of EPV in the important 600-500 mb layer for this case. Originally F33, strongest frontal zone was across southwest lower Michigan/far northwest Indiana in drier low level air. At F21, stronger frontogenesis and resultant negative EPV begins to shift southward, but too far south. Not accounting for rotational frontogenesis? 00Z NAM depiction of instability better matches observed heaviest snowfall. Orientation of strongest baroclincity altered by rotational frontogenesis likely responsible.33

Mesoscale/synoptic forcing mechanisms not well modeled 18 to 36 hours in advance of heavy snow band.

Ingredients plots did provided some indication of banding potential.

Smaller scale synoptic forcing acted to reinforce/focus mesoscale banding.

Q-vector partition indicated that synoptic scale forcing likely played large role in rotational frontogenesis, extended duration mid/level forcing from previous forecast thinking.

Strong indications of slantwise ascent/CSI.

QG analysis from later runs of NAM appeared to align well with mesoscale band.

Event serves an important reminder of the synoptic/mesoscale link.

SUMMARYSummary of the event with key factors being 1) importance of synoptic scale governing mesoscale 2) type of flow/high deformation made QG diagnostics more applicable than they normally might be.34