Cold Air Damming: An Introduction Gail Hartfield NWSFO Raleigh, North Carolina

By the end of this instruction, you should be able to... 4 List each damming type, and describe the relative roles of synoptic scale forcing & diabatic processes in each. 4 Describe the major influencing processes of damming, both at the surface and aloft, & explain their effects. 4 Discern between damming and lookalike (non- damming) events.

Why Study Cold Air Damming? 4 It happens often & affects a wide area 4 Models don’t diagnose/forecast it well… even mesoscale models have trouble 4 Occurrence has major implications on cloud cover, temps, precip type, etc. 4 Not all ridges down the East Coast are “damming”!

I live in Juneau. What do I care? 4 Cold air damming occurs often east of the Rockies, too… not just the Appalachians 4 Many of the processes affecting damming are noted in other phenomena as well 4 The need for a thorough understanding of contributing processes is applicable to any forecast problem 4 An event-specific forecast process can be useful for many forecast problems 4 You aren’t glued to your current station!

Forecast Challenges of the Mid-Atlantic & Southeast U.S Often at southern extent of cold air 4 Highest mountains in the Appalachians 4 Gulf Stream & Atlantic in close proximity 4 Extensive Piedmont & Coastal Plain

The Damming Region (DR) 4 Area under greatest consideration for “spectrum” 4 Damming dome deepest 4 Is by no means the only area affected!

Spectrum of Cold Air Damming and Lookalikes... 4 …is a method of classifying events based on processes 4 …was created to help forecasters identify the very different types of damming events 4 …helps with coordination 4 …will continue to be adjusted and improved as more is learned

4 Five types: 3 damming 2 lookalikes 4 Damming=> BLOCKED FLOW

Froude Number H = height of mtn barrier U = component of mean wind orthogonal to mtns  = mean value through stable layer h = height of stable layer = (Brunt-Vaisala frequency)

4 Spectrum is a continuum 4 All produce same weather conditions 4 Lookalikes=> UNBLOCKED FLOW 4 Damming=> BLOCKED FLOW 4 Five types: 3 damming 2 lookalikes

“Classical” Cold Air Damming 4 Strong forcing from synoptic-scale features 4 Diabatic processes unnecessary to initiate, but can strengthen 4 Note position and strength of sfc high

Surface Processes of Classical Cold Air Damming 4 “Parent” high is cold air source 4 E to NE flow is blocked & deflected southward 4 Adiabatic cooling=> hydrostatic pressure rise=> ageostrophic response 4 CAA & low level stability in DR are enhanced

Diabatic processes become more important... Synoptic-scale forcing becomes less important However, in the non-classical damming types…

Hybrid Damming 4 Synoptic-scale forcing & diabatic processes play nearly equal roles 4 Parent high may be: ûIn good position but weak ûProgressive (limited CAA) 4 Strong signatures aloft often lacking 4 Diabatic processes enhance low-level stability

In Situ Damming 4 Diabatic processes necessary 4 Little or no CAA initially; cool dry air is deposited 4 Sfc high is unfavorably located 4 Precip into this pre-existing dry, stable air instigates damming

In Situ Damming Event: 6-7 Jan Temperatures were in the lower 60s in Eastern NC & lower 30s in Central NC 4 Boundaries can be focus for severe weather (more later) 4 Millions of dollars in damage in NC alone; >120 kt gust at GSB

A Brief Look at the “Lookalikes” 4 Weather conditions mimic cold air damming 4 Differs from damming… '  Flow is NOT blocked '  Not connected to a parent high '  Lacks signatures above the boundary layer 4 Two types: Cool air pooling & upslope

Cool Air Pooling 4 Pre-existing dry air mass not connected to a parent high 4 No CAA into cool pool 4 Precipitation induces mesoscale high 4 Mountains not required 4 CAD events frequently turn into cool air pooling!

Upslope Flow 4 Adiabatic lift generates considerable cloudiness & cooler temperatures 4 Resulting surface meso- high has no connection to or support by a parent high 4 Low-levels too unstable for damming 00Z 10/14/95 Boundary Layer Surface

To recap the damming types… 4 Classical = support & forcing from synoptic- scale features, surface & aloft; diabatic processes not needed 4 Hybrid = support & forcing from both synoptic-scale features & diabatic processes 4 In Situ = instigated by diabatic processes with little or no support from synoptic-scale features

Processes Aloft Contributing to Cold Air Damming 4 Can effect near-surface environment significantly 4 Notable mainly in classical and sometimes hybrid CAD 4 Contributing processes evident at: à850 mb à500 mb à mb

CAD Processes & Signatures: 850 mb äGenerates clouds & precip for increased stability 4 Anticyclone off SE U.S. coast äStrengthens inversion 4 Enhances CAD: 4 Light-moderate warm moist flow atop cold dome

CAD Processes & Signatures: 500 mb 4 Allows surface ridge to be unimpeded by cyclogenesis 4 Split-flow regime 4 Confluent flow over NE U.S. anchors & strengthens high 'Trough or low over Ern Canada 'Trough or low in Srn Plains

4 Ageostrophic circulation… CAD Processes & Signatures: 300 mb äHelps drive sfc cold air southward 4 Jet entrance region is over NE U.S. äProduces subsidence atop sfc high

Cold Air Damming Erosion (or, When is this “dam” thing gonna end??) 4 One of the most difficult aspects of CAD, not captured well by models 4 Incorporate model biases in forecast process (e.g. NGM moves parent highs offshore too quickly) 4 Rules of thumb: +Strong events typically require strong CFP to scour out wedge (esp. Oct-Mar) +Weak events with only low cloud cover are susceptible to erosion by insolation & mixing from above

Erosion & Breakdown: A Few Questions to Ask 4 Is low level CAA ending? (e.g. parent high moving offshore; being “pinched off”) 4 Are surface winds shifting out of damming configuration? 4 Is upper level support waning? 4 Is precipitation ending (influence of diabiatic processes diminishing)? 4 Has dry air advection ended? 4 Could this event end as cool air pooling?

Cold Air Damming: Forecast Operations 4 Tools for identifying an event & diagnosing the influencing processes  Spectrum of Damming and Lookalike Events Glossary of Terms For CAD & Lookalikes Special AWIPS procedures Forecast Methodology for CAD 4 Tools for determining CAD onset and erosion  Models (e.g. Eta, MASS, MM5) (longer term) Close monitoring of sfc/BL/UA features CAD Erosion Guidelines (in progress) Conceptual models

“Forecast Methodology for CAD” 4 Created to facilitate event identification and the forecast process 4 Adapted for online use w/ MASS model (but is also in questionnaire format) 4 Three parts: #Pre-Development (Is the stage set?) #Development (assessment/ID; is flow blocked?) #Breakdown & Erosion (identify possible mechanisms of wedge erosion)

“Pre-Development” 4 Links to pertinent MASS & Eta model fields 4 Addresses: 'Sfc high initial position, strength & source 'Sfc temps/dewpoints 'Availability of dry air, & dry air ridge (DAR) development

“Development” 4 Links to MASS, NGM, & Eta fields 4 Addresses: 'Low level CAA 'Upper level support (850/500/300 mb) 'Low level stability

“Breakdown & Erosion” 4 In “yes/no” questionnaire format 4 Addresses: 'Cessation of diabatic processes, low level CAA, upper level support, sfc high support 'Presence of thermal- moisture boundaries (TMBs)

Thermal-Moisture Boundaries (aka wedge fronts, piedmont fronts) 4 Delineate the southern and eastern edges of the cold dome 4 Temp differences across TMB are often 20  F or greater 4 Coastal front can “jump” inland into TMB  4 Can act as a focus for severe weather 

Coastal Fronts 4 Development favored by: ûVery cold air over warm Gulf Stream ûPre-existing synoptic frontal boundary ûDifferential heating ûConvergence zone 4 Onshore movement indicated by: ûOffshore NE winds go SE (check buoy obs) ûTight thermal gradient pushing westward ûPressure falls & temp/dewpoint rises just inland

Coastal Fronts 4 Factors affecting inland movement or “jump”: ûStrength of wedge ûDepth of cold dome on edges ûOffshore high pressure with sufficiently strong southeasterly flow orthogonal to front ûStrong/strengthening TMB + weakening coastal front 4 Will not likely move much farther west than Raleigh/Burlington, NC

Severe Weather Along a TMB 4 Strong vertical shear along TMB enhances severe threat 4 Type of damming can determine degree of threat û Severe wx more likely with in situ damming 4 Cold front aloft (CFA) & accompanying dry slot can enhance severe downdrafts 4 Check presence of low level jet streak

To wrap it up... 4 CAD mustn’t be oversimplified... the relative roles of various processes differ in each event 4 Forecasters must understand the supporting processes of each event & recognize the signatures 4 Forecast methodologies targeting particular weather problems (e.g. landfalling TCs, heavy snow QPF) can make the entire forecast process easier and more efficient 4 CAD boundaries can spawn severe weather